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

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: https://github.com/ClangBuiltLinux/linux/issues/2109
Fixes: ca1a66cdd685 ("riscv: uaccess: do not do misaligned accesses in get/put_user()")
Link: https://github.com/llvm/llvm-project/commit/2464313eef01c5b1edf0eccf57a32cdee01472c7
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>