From: Neil Brown <neilb@cse.unsw.edu.au>

Fix bug #2661

Raid currently calls ->unplug_fn under spin_lock_irqsave(), but unplug_fns
can sleep.


After a morning of scratching my head and trying to come up with some that
does less locking, the following is the best I can come up with.  I'm not
proud of it but it should work.

If I move "nr_pending" out or rdev into the per-personality structures
(e.g.  mirror_info), and if I had "atomic_inc_if_nonzero" I could do with
without locking so much, but random atomic* functions don't seem trivial