[SCSI] Add ability to scan scsi busses asynchronously

Since it often takes around 20-30 seconds to scan a scsi bus, it's
highly advantageous to do this in parallel with other things.  The bulk
of this patch is ensuring that devices don't change numbering, and that
all devices are discovered prior to trying to start init.  For those
who build SCSI as modules, there's a new scsi_wait_scan module that will
ensure all bus scans are finished.

This patch only handles drivers which call scsi_scan_host.  Fibre Channel,
SAS, SATA, USB and Firewire all need additional work.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>