bitfield.h: Fix "type of reg too small for mask" test

The test: 'mask > (typeof(_reg))~0ull' only works correctly when both
sides are unsigned, consider:

 - 0xff000000 vs (int)~0ull
 - 0x000000ff vs (int)~0ull

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20211110101324.950210584@infradead.org

Author: Peter Zijlstra

include/linux/bitfield.h

@@ -41,6 +41,22 @@
 
 #define __bf_shf(x) (__builtin_ffsll(x) - 1)
 
+#define __scalar_type_to_unsigned_cases(type)				\
+		unsigned type:	(unsigned type)0,			\
+		signed type:	(unsigned type)0
+
+#define __unsigned_scalar_typeof(x) typeof(				\
+		_Generic((x),						\
+			char:	(unsigned char)0,			\
+			__scalar_type_to_unsigned_cases(char),		\
+			__scalar_type_to_unsigned_cases(short),		\
+			__scalar_type_to_unsigned_cases(int),		\
+			__scalar_type_to_unsigned_cases(long),		\
+			__scalar_type_to_unsigned_cases(long long),	\
+			default: (x)))
+
+#define __bf_cast_unsigned(type, x)	((__unsigned_scalar_typeof(type))(x))
+
 #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx)			\
 	({								\
 		BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),		\
@@ -49,7 +65,8 @@
 		BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?		\
 				 ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
 				 _pfx "value too large for the field"); \
-		BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ull,		\
+		BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) >	\
+				 __bf_cast_unsigned(_reg, ~0ull),	\
 				 _pfx "type of reg too small for mask"); \
 		__BUILD_BUG_ON_NOT_POWER_OF_2((_mask) +			\
 					      (1ULL << __bf_shf(_mask))); \