mpt2sas: Add free smids to the head, not tail of list

mpt2sas: Add free smids to the head, not tail of list

Reusing a smid quickly means it's still cache-hot.  This yields a small
but noticable performance improvement in a well-known database benchmark.
This improvement is already present in the mpt3sas driver.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Reviewed-by: Nicholas Bellinger <nab@linux-iscsi.org>
Reviewed-by: Praveen Krishnamoorthy <Praveen.krishnamoorthy@lsi.com>
Acked-by: Sreekanth Reddy <Sreekanth.reddy@lsi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>