[SCSI] add missing scan mutex to scsi_scan_target()

This patch (as543) adds a private entry point to scsi_scan_target, for use
when the caller already owns the scan_mutex, and updates the kerneldoc for
that routine (which was badly out-of-date).  It converts scsi_scan_channel
to use the new entry point.  Lastly, it modifies scsi_get_host_dev to make
it acquire the scan_mutex, necessary since the routine adds a new
scsi_device even if it doesn't do any actual scanning.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>