[SCSI] libfcoe: fcoe_ctlr_destroy use cancel_work_sync instead of flush_work

Use cancel_work_sync() in place of flush_work(), so that
fcoe_ctlr_destroy() can be called from a workqueue.

Also, purge the receive queue after the recv_work has been cancled because
if recv_work isn't run it's not guaranteed to be empty now.

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>