fc: ensure scan_work isn't active when freeing fc_rport

debugfs caught this:
WARNING: at lib/debugobjects.c:260 debug_print_object+0x83/0xa0()
ODEBUG: free active (active state 0) object type: work_struct
hint: fc_scsi_scan_rport+0x0/0xd0 [scsi_transport_fc]
 CPU: 1 PID: 184 Comm: kworker/1:1 Tainted: G        W
--------------   3.10.0-123.el7.x86_64.debug #1
Hardware name: HP ProLiant DL120 G7, BIOS J01 07/01/2013
Workqueue: fc_wq_5 fc_rport_final_delete [scsi_transport_fc]
Call Trace:
[<ffffffff8169efec>] dump_stack+0x19/0x1b
[<ffffffff8106cbd1>] warn_slowpath_common+0x61/0x80
[<ffffffff8106cc4c>] warn_slowpath_fmt+0x5c/0x80
[<ffffffff8133e003>] debug_print_object+0x83/0xa0
[<ffffffffa04e2f40>] ? fc_parse_wwn+0x100/0x100

[<ffffffff8133f23b>] debug_check_no_obj_freed+0x22b/0x270
[<ffffffffa04e127e>] ? fc_rport_dev_release+0x1e/0x30
[<ffffffff811db3e9>] kfree+0xd9/0x2d0
[<ffffffffa04e127e>] fc_rport_dev_release+0x1e/0x30
[<ffffffff81428032>] device_release+0x32/0xa0
[<ffffffff8132701e>] kobject_release+0x7e/0x1b0
[<ffffffff81326ed8>] kobject_put+0x28/0x60
[<ffffffff81428397>] put_device+0x17/0x20
[<ffffffffa04e5025>] fc_rport_final_delete+0x165/0x210
[<ffffffff810959b0>] process_one_work+0x220/0x710
[<ffffffff81095944>] ? process_one_work+0x1b4/0x710
[<ffffffff81095fbb>] worker_thread+0x11b/0x3a0
[<ffffffff81095ea0>] ? process_one_work+0x710/0x710
[<ffffffff8109e0cd>] kthread+0xed/0x100
[<ffffffff8109dfe0>] ? insert_kthread_work+0x80/0x80
[<ffffffff816b2fec>] ret_from_fork+0x7c/0xb0
[<ffffffff8109dfe0>] ? insert_kthread_work+0x80/0x80

Seems to be because the scan_work work_struct might be active when the housing
fc_rport struct gets freed.  Ensure that we cancel it prior to freeing the rport

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reviewed-by: Vasu Dev <vasu.dev@intel.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>