[SCSI] fcoe: use rtnl mutex in place of hostlist lock

This just cuts down on the number of locks we're dealing with, and
eliminates the need to take another lock in the netdev notifier.

Signed-off-by: Chris Leech <christopher.leech@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>