mpt3sas: Refcount sas_device objects and fix unsafe list usage

mpt3sas: 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.

This patch is a port of Calvin's PATCH-v4 for mpt2sas code, atop
mpt3sas changes in scsi.git/for-next.

Cc: Calvin Owens <calvinowens@fb.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Sreekanth Reddy <sreekanth.reddy@avagotech.com>
Cc: MPT-FusionLinux.pdl <MPT-FusionLinux.pdl@avagotech.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>