The fw_event_work struct is concurrently referenced at shutdown. Add a
refcount to protect it and refactor the code to use it.

Additionally, refactor _scsih_fw_event_cleanup_queue() such that it no
longer iterates over the list without holding the lock since
_firmware_event_work() concurrently deletes items from the list.

This patch is ported from commit 008549f6 ("mpt2sas: Refcount
fw_events and fix unsafe list usage"). These changes are also required
for mpt3sas.
Signed-off-by: NSreekanth Reddy <Sreekanth.Reddy@avagotech.com>
Acked-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NHannes Reinecke <hare@suse.de>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>