iommupt: Make it clearer to the compiler that pts.level == 0 for single page

Older versions of gcc and clang sometimes get tripped up by the build time
assertion in FIELD_PREP because they can see that the argument to
FIELD_PREP is constant but can't see that the if condition protecting it
is also a constant false.

   In file included from <command-line>:
   In function 'amdv1pt_install_leaf_entry',
       inlined from '__do_map_single_page' at drivers/iommu/generic_pt/fmt/../iommu_pt.h:651:3,
       inlined from '__map_single_page0' at drivers/iommu/generic_pt/fmt/../iommu_pt.h:662:1,
       inlined from 'pt_descend' at drivers/iommu/generic_pt/fmt/../pt_iter.h:391:9,
       inlined from '__do_map_single_page' at drivers/iommu/generic_pt/fmt/../iommu_pt.h:658:10,
       inlined from '__map_single_page1.constprop' at drivers/iommu/generic_pt/fmt/../iommu_pt.h:662:1:
   ././include/linux/compiler_types.h:631:45: error: call to '__compiletime_assert_251' declared with attribute error: FIELD_PREP: value too large for the field
     631 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
	 |                                             ^
   ././include/linux/compiler_types.h:612:25: note: in definition of macro '__compiletime_assert'
     612 |                         prefix ## suffix();                             \
	 |                         ^~~~~~
   ././include/linux/compiler_types.h:631:9: note: in expansion of macro '_compiletime_assert'
     631 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
	 |         ^~~~~~~~~~~~~~~~~~~
   ./include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
      39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
	 |                                     ^~~~~~~~~~~~~~~~~~
   ./include/linux/bitfield.h:69:17: note: in expansion of macro 'BUILD_BUG_ON_MSG'
      69 |                 BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?           \
	 |                 ^~~~~~~~~~~~~~~~
   ./include/linux/bitfield.h:90:17: note: in expansion of macro '__BF_FIELD_CHECK_MASK'
      90 |                 __BF_FIELD_CHECK_MASK(mask, val, pfx);                  \
	 |                 ^~~~~~~~~~~~~~~~~~~~~
   ./include/linux/bitfield.h:137:17: note: in expansion of macro '__FIELD_PREP'
     137 |                 __FIELD_PREP(_mask, _val, "FIELD_PREP: ");              \
	 |                 ^~~~~~~~~~~~
   drivers/iommu/generic_pt/fmt/amdv1.h:220:26: note: in expansion of macro 'FIELD_PREP'
     220 |                          FIELD_PREP(AMDV1PT_FMT_OA,
	 |                          ^~~~~~~~~~

Changing the caller to check pts.level == 0 avoids demanding a bit of
complex reasoning from the compiler that pts.level == level == 0. Instead
the compiler sees that pt_install_leaf_entry() is called with a constant
pts.level == 0 which makes it more reliable to see the constant false in
the if.

Fixes: dcd6a01 ("iommupt: Add map_pages op")
Reported-by: Chunyu Hu <chuhu@redhat.com>
Closes: https://lore.kernel.org/all/aUn9uGPCooqB-RIF@gmail.com/
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

Author: jgunthorpe

drivers/iommu/generic_pt/iommu_pt.h

@@ -645,7 +645,7 @@ static __always_inline int __do_map_single_page(struct pt_range *range,
 	struct pt_iommu_map_args *map = arg;
 
 	pts.type = pt_load_single_entry(&pts);
-	if (level == 0) {
+	if (pts.level == 0) {
 		if (pts.type != PT_ENTRY_EMPTY)
 			return -EADDRINUSE;
 		pt_install_leaf_entry(&pts, map->oa, PAGE_SHIFT,