P-RES: allocation in pres_admit_task() must be atomic

The kernel codepath calling into pres_admit_task() is holding some
lock unrelated to LITMUS^RT. As a result, we need to pass GFP_ATOMIC,
not just GFP_KERNEL, to kzalloc().
P-RES: disable LITMUS^RT's standard budget enforcement

The P-RES plugin is currently not compatible with the per-thread
budget enforcement logic, which can trigger assertion failures. For
now, let's simply disable per-thread timeslice enforcement. (P-RES's
reservations are a much better mechanism anyway.)
P-RES: keep track in per-task state of whether it suspended

Checking state->scheduled is not accurate when bandwidth inheritance
is applied.