mpt2sas: Refcount sas_device objects and fix unsafe list usage

These objects can be referenced concurrently throughout the driver, we
need a way to make sure threads can't delete them out from under each
other. This patch adds the refcount, and refactors the code to use it.

Additionally, we cannot iterate over the sas_device_list without
holding the lock, or we risk corrupting random memory if items are
added or deleted as we iterate. This patch refactors _scsih_probe_sas()
to use the sas_device_list in a safe way.

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>