Fix for unbalanced EPOCH(9) usage in the generic kernel interrupt
handler.

Interrupt handlers are removed via intr_event_execute_handlers() when
IH_DEAD is set. The thread removing the interrupt is woken up, and
calls intr_event_update(). When this happens, the ie_hflags are
cleared and re-built from all the remaining handlers sharing the
event. When the last IH_NET handler is removed, the IH_NET flag will
be cleared from ih_hflags (or ie_hflags may still be being rebuilt in
a different context), and the ithread_execute_handlers() may return
with ie_hflags missing IH_NET. This can lead to a scenario where
IH_NET was present before calling ithread_execute_handlers, and is not
present at its return, meaning the need for epoch must be cached
locally.

This can happen when loading and unloading network drivers. Also make
sure the ie_hflags is not cleared before being updated.

This is a regression issue after r357004.

Backtrace:
panic()

trying to access epoch tracker on stack of dead thread

_epoch_enter_preempt()
ifunit_ref()
ifioctl()
fo_ioctl()
kern_ioctl()
sys_ioctl()
syscallenter()
amd64_syscall()

Differential Revision: https://reviews.freebsd.org/D23483
Reviewed by: glebius@, gallatin@, mav@, jeff@ and kib@
Sponsored by: Mellanox Technologies