[SCSI] libfcoe: clean up netdev mapping properly when the transport goes away

When rmmoving the underlying fcoe transport driver module by force when
it's attached and in use, the correspoding netdev mapping should be
cleaned up properly as well, otherwise the lookup for a given netdev
for the transport would still return non NULL pointer, causing "unable
to handle paging request" bug.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>