Change all syscall function name prefix to __sys

Instead of using uring_mmap, uring_close, uring_madvise, etc. Let's
use __sys_mmap, __sys_close, __sys_madvise, etc. That looks better
convention for syscall function name like what we do with
__sys_io_uring* functions.

Cc: Nugra <richiisei@gmail.com>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Link: https://lore.kernel.org/r/20220211155753.143698-5-ammarfaizi2@gnuweeb.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>