sched/rt: Move group schedulability check to sched_rt_global_validate()

sched/rt: Move group schedulability check to sched_rt_global_validate()

The sched_rt_global_constraints() function is a remnant that used to set
up global RT throttling but that is no more since commit 5f6bd380c7bdb
("sched/rt: Remove default bandwidth control") and the function ended up
only doing schedulability check.
Move the check into the validation function where it fits better.
(The order of validations sched_dl_global_validate() and
sched_rt_global_validate() shouldn't matter.)

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260323-sched-rert_groups-v3-2-1e7d5ed6b249@suse.com