io_uring: fix compile warning for 32-bit builds

If IO_URING_SCM_ALL isn't set, as it would not be on 32-bit builds,
then we trigger a warning:

fs/io_uring.c: In function '__io_sqe_files_unregister':
fs/io_uring.c:8992:13: warning: unused variable 'i' [-Wunused-variable]
 8992 |         int i;
      |             ^

Move the ifdef up to include the 'i' variable declaration.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 5e45690a ("io_uring: store SCM state in io_fixed_file->file_ptr")
Signed-off-by: Jens Axboe <axboe@kernel.dk>