sched: Move attach_one_task and attach_task helpers to sched.h

sched: Move attach_one_task and attach_task helpers to sched.h

The fair scheduler locally introduced attach_one_task() and
attach_task() helpers, but these could be generically useful so
move this code to sched.h so we can use them elsewhere.

One minor tweak made to utilize guard(rq_lock)(rq) to simplifiy
the function.

Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://patch.msgid.link/20260324191337.1841376-10-jstultz@google.com