scsi: fcoe: fix null-ptr-deref Read in fc_release_transport

In fcoe_if_init, if fc_attach_transport(&fcoe_vport_fc_functions)
fails, need to free the previously memory and return fail, otherwise
will trigger null-ptr-deref Read in fc_release_transport.

fcoe_exit
  fcoe_if_exit
    fc_release_transport(fcoe_vport_scsi_transport)

Link: https://lore.kernel.org/r/1566279789-58207-1-git-send-email-zhengbin13@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>