sched/core: Provide sched_rtmutex() and expose sched work helpers

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2218724

Upstream Status: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git

Avoid corrupting lock state due to blocking on a lock in sched_submit_work() while in the process of blocking on another lock.

Signed-off-by: Crystal Wood swood@redhat.com