riscv: Add intermediate cast to 'unsigned long' in __get_user_asm

After commit bdce162 ("riscv: Use 64-bit variable for output in
__get_user_asm"), there is a warning when building for 32-bit RISC-V:

  In file included from include/linux/uaccess.h:13,
                   from include/linux/sched/task.h:13,
                   from include/linux/sched/signal.h:9,
                   from include/linux/rcuwait.h:6,
                   from include/linux/mm.h:36,
                   from include/linux/migrate.h:5,
                   from mm/migrate.c:16:
  mm/migrate.c: In function 'do_pages_move':
  arch/riscv/include/asm/uaccess.h:115:15: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
    115 |         (x) = (__typeof__(x))__tmp;                             \
        |               ^
  arch/riscv/include/asm/uaccess.h:198:17: note: in expansion of macro '__get_user_asm'
    198 |                 __get_user_asm("lb", (x), __gu_ptr, label);     \
        |                 ^~~~~~~~~~~~~~
  arch/riscv/include/asm/uaccess.h:218:9: note: in expansion of macro '__get_user_nocheck'
    218 |         __get_user_nocheck(x, ptr, __gu_failed);                        \
        |         ^~~~~~~~~~~~~~~~~~
  arch/riscv/include/asm/uaccess.h:255:9: note: in expansion of macro '__get_user_error'
    255 |         __get_user_error(__gu_val, __gu_ptr, __gu_err);         \
        |         ^~~~~~~~~~~~~~~~
  arch/riscv/include/asm/uaccess.h:285:17: note: in expansion of macro '__get_user'
    285 |                 __get_user((x), __p) :                          \
        |                 ^~~~~~~~~~
  mm/migrate.c:2358:29: note: in expansion of macro 'get_user'
   2358 |                         if (get_user(p, pages + i))
        |                             ^~~~~~~~

Add an intermediate cast to 'unsigned long', which is guaranteed to be the same
width as a pointer, before the cast to the type of the output variable to clear
up the warning.

Fixes: bdce162 ("riscv: Use 64-bit variable for output in __get_user_asm")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202601210526.OT45dlOZ-lkp@intel.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260121-riscv-fix-int-to-pointer-cast-v1-1-b83eebe57c76@kernel.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: nathanchance

arch/riscv/include/asm/uaccess.h

@@ -112,7 +112,7 @@ do {								\
 		_ASM_EXTABLE_UACCESS_ERR(1b, %l2, %0)		\
 		: "=&r" (__tmp)					\
 		: "m" (*(ptr)) : : label);			\
-	(x) = (__typeof__(x))__tmp;				\
+	(x) = (__typeof__(x))(unsigned long)__tmp;		\
 } while (0)
 #else /* !CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
 #define __get_user_asm(insn, x, ptr, label)			\