x86: Restore the critical section around whole ipi_bitmap_handler() if
hardclock IPI is delivered.

In the current code after r355311, critical section is taken only
around hardclockintr() call, and sched_preempt() is called after the
section is exited. If we reschedule after exit, as we typically would
due to conditions that caused IPI, in ULE the runq tdq_ipipending is
not cleared, which blocks generation of further preempt IPIs.

Since all relatively modern (10 years) hardware has per-cpu event
timers, restoring the critical section conditionally does not affect
it.

Reported and tested by: cy
Diagnosed and reviewed by: jeff (previous version)
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D22716