mpt2sas: Refcount fw_events and fix unsafe list usage

The fw_event_work struct is concurrently referenced at shutdown, so
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.

Signed-off-by: Calvin Owens <calvinowens@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nicholas Bellinger <nab@linux-iscsi.org>
Tested-by: Chaitra Basappa <chaitra.basappa@avagotech.com>
Acked-by: Sreekanth Reddy <sreekanth.reddy@avagotech.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>