sched_ext: Make scx_kf_allowed_if_unlocked() available outside ext.c

Relocate the scx_kf_allowed_if_unlocked(), so it can be used from other
source files (e.g., ext_idle.c).

No functional change.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>