MFC: r236579

MFC: r236579

The workaround added in r151650 for handling firmwares that don't allow
a single device to be opened multiple times concurrently unfortunately
isn't sufficient with ZFS. This is due to the fact, that ZFS may open
different partitions of a single device simultaneously. So the best we
can do in this case is to cache the lastly used device path and close
and open devices in ofwd_strategy() as needed.

PR:		165025
Submitted by:	Gavin Mu

git-svn-id: svn://svn.freebsd.org/base/stable/9@237091 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f