linux(4): Add a dedicated ioprio system calls

On Linux these system calls have an effect only when used in conjuction
with an I/O scheduler that supports I/O priorities. If no I/O scheduler
has been set for a thread, then by defaut the I/O priority will follow
the CPU nice value. Due to FreeBSD lack of I/O scheduler facilities, the
default Linux behavior is implemented.

Ubuntu 23.04 debootstrap requires Linux ionice which depends on these
syscalls.

Differential Revision: https://reviews.freebsd.org/D41153
MFC after: 1 month