[SCSI] mpt fusion: Added support for MPT discovery completion check

sas_discovery_quiesce_io flag is used to control IO start/resume functionality.
IO will be stoped while doing discovery of topology. Once discovery is completed
It will resume IO. Resending patch including James review.

Signed-off-by: Kashyap Desai <kadesai@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>