riscv: uaccess: use input constraints for ptr of __put_user()

Putting ptr in the inputs as opposed to output may seem incorrect but
this is done for a few reasons:
- Not having it in the output permits the use of asm goto in a
  subsequent patch. There are bugs in gcc [1] which would otherwise
  prevent it.
- Since the output memory is userspace there isn't any real benefit from
  telling the compiler about the memory clobber.
- x86, arm and powerpc all use this technique.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113921

 # 1

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
[Cyril Bur: Rewritten commit message]
Signed-off-by: Cyril Bur <cyrilbur@tenstorrent.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250410070526.3160847-4-cyrilbur@tenstorrent.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>