RISC-V: uaccess: Wrap the get_user_8 uaccess macro

RISC-V: uaccess: Wrap the get_user_8 uaccess macro

I must have lost this rebasing things during the merge window, I know I
got it at some point but it's not here now.  Without this I get warnings
along the lines of

    include/linux/fs.h:3975:15: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
     3975 |         if (unlikely(get_user(c, path)))
          |                      ^
    arch/riscv/include/asm/uaccess.h:274:3: note: expanded from macro 'get_user'
      274 |                 __get_user((x), __p) :                          \
          |                 ^
    arch/riscv/include/asm/uaccess.h:244:2: note: expanded from macro '__get_user'
      244 |         __get_user_error(__gu_val, __gu_ptr, __gu_err);         \
          |         ^
    arch/riscv/include/asm/uaccess.h:207:2: note: expanded from macro '__get_user_error'
      207 |         __ge  LD [M]  net/802/psnap.ko
    t_user_nocheck(x, ptr, __gu_failed);                        \
          |         ^
    arch/riscv/include/asm/uaccess.h:196:3: note: expanded from macro '__get_user_nocheck'
      196 |                 __get_user_8((x), __gu_ptr, label);             \
          |                 ^
    arch/riscv/include/asm/uaccess.h:130:2: note: expanded from macro '__get_user_8'
      130 |         u32 __user *__ptr = (u32 __user *)(ptr);                \
          |         ^

Link: https://lore.kernel.org/r/20250610213058.24852-1-palmer@dabbelt.com
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: stable@vger.kernel.org
Fixes: f6bff7827a48 ("riscv: uaccess: use 'asm_goto_output' for get_user()")
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>