kernel - Fix rare IPIQ freezes
* Ensure that an IPI interrupt is sent went waiting for an IPIQ
to drain. The IPIQ can be pushed up by passive IPIs and not
necessarily have a signal pending on the target cpu, so we
have to put the check in our drain loop rather than outside
the loop.
* Add a cpu_pause() to reduce power use for the IPIQ drain case.
* Normalize the use of gd_intr_nesting_level and clean up the
code syntax for the ipiq_optimized case.
* Remove the previous temporary IPI interrupt signaling fix, it
was incomplete.
* Fix a missing crit_exit() in the ENOENT case for
lwkt_send_ipiq3_nowait().
* Track cpu's which are in the middle of ipiq processing and
assert that a cpu is not in an IPIQ processing loop when
switching between threads.
* Normalize the use of ip->ip_npoll in the IPIQ code. This
field is used to avoid unnecessary IPI interrupts.