riscv: uaccess: Fix -Wuninitialized and -Wshadow in __put_user_nocheck

After a recent change in clang to strengthen uninitialized warnings [1],
there is a warning from val being uninitialized in __put_user_nocheck
when called from futex_put_value():

  kernel/futex/futex.h:326:18: warning: variable 'val' is uninitialized when used within its own initialization [-Wuninitialized]
    326 |         unsafe_put_user(val, to, Efault);
        |         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
  arch/riscv/include/asm/uaccess.h:464:21: note: expanded from macro 'unsafe_put_user'
    464 |         __put_user_nocheck(x, (ptr), label)
        |         ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
  arch/riscv/include/asm/uaccess.h:314:36: note: expanded from macro '__put_user_nocheck'
    314 |                 __inttype(x) val = (__inttype(x))x;                     \
        |                              ~~~                 ^

While not on by default, -Wshadow flags the same mistake:

  kernel/futex/futex.h:326:2: warning: declaration shadows a local variable [-Wshadow]
    326 |         unsafe_put_user(val, to, Efault);
        |         ^
  arch/riscv/include/asm/uaccess.h:464:2: note: expanded from macro 'unsafe_put_user'
    464 |         __put_user_nocheck(x, (ptr), label)
        |         ^
  arch/riscv/include/asm/uaccess.h:314:16: note: expanded from macro '__put_user_nocheck'
    314 |                 __inttype(x) val = (__inttype(x))x;                     \
        |                              ^
  kernel/futex/futex.h:320:48: note: previous declaration is here
    320 | static __always_inline int futex_put_value(u32 val, u32 __user *to)
        |                                                ^

Use a three underscore prefix for the val variable in __put_user_nocheck
to avoid clashing with either val or __val, which are both used within
the put_user macros, clearing up all warnings.

Closes: ClangBuiltLinux#2109
Fixes: ca1a66c ("riscv: uaccess: do not do misaligned accesses in get/put_user()")
Link: llvm/llvm-project@2464313 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250715-riscv-uaccess-fix-self-init-val-v1-1-82b8e911f120@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

Author: nathanchance

arch/riscv/include/asm/uaccess.h

@@ -311,8 +311,8 @@ do {								\
 do {								\
 	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&	\
 	    !IS_ALIGNED((uintptr_t)__gu_ptr, sizeof(*__gu_ptr))) {	\
-		__inttype(x) val = (__inttype(x))x;			\
-		if (__asm_copy_to_user_sum_enabled(__gu_ptr, &(val), sizeof(*__gu_ptr))) \
+		__inttype(x) ___val = (__inttype(x))x;			\
+		if (__asm_copy_to_user_sum_enabled(__gu_ptr, &(___val), sizeof(*__gu_ptr))) \
 			goto label;				\
 		break;						\
 	}							\