[SCSI] libfc: release exchg cache

If fail to create workqueue, the newly created cache for exchg has to be
released.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Reviewed-by: Vasu Dev <vasu.dev@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>