Fix data race between zil_commit() and zil_suspend()

openzfsonwindows/openzfs#206 found that it is possible to trip
VERIFY(list_is_empty(&lwb->lwb_itxs)) when a zil_commit() is delayed
by the scheduler long enough for a parallel zil_suspend() operation to
exit zil_commit_impl(). This is a data race. To prevent this, we
introduce a zilog->zl_suspend_lock rwlock to ensure that all
outstanding zil_commit() operations finish before zil_suspend()
begins and that subsequent operations fallback to txg_wait_synced()
after zil_suspend() has begun.

On PREEMPT_RT Linux kernels, the rw_enter() implementation suffers
from writer starvation. This means that a ZIL intensive system can delay
zil_suspend() indefinitely. This is a pre-existing problem that
affects everything that uses rw locks, so it needs to be addressed in
the SPL. However, builds against PREEMPT_RT Linux kernels are
currently broken due to a GPL symbol issue (#11097), so we can safely
disregard that issue for now.

Reported-by: Arun KV <arun.kv@datacore.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14514