[SCSI] libsas: Destroy the task collector thread after releasing ports

If we use task collector mode, we can end up destroying the task collector
thread before we release the ports, which is bad if a port release causes
a disk I/O (such as cache flushing).

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>