Add support for async scanning

Notes: This is the async scan patch to our driver from Matthew Wilcox.
  The async scan logic is still subject to errors in insmod/rmmod, as
  the async scan threads don't get shutdown when the module unloads
  underneath them. See http://marc.info/?l=linux-scsi&m=117551999925582&w=2Signed-off-by: NJames Smart <James.Smart@emulex.com>
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>