tcm_fc: Wait for command completion before freeing a session

This patch avoids that the following kernel crash can occur with
later patches in this patch series:

general protection fault: 0000 [#1] SMP
CPU: 0 PID: 6 Comm: kworker/u8:0 Not tainted 4.3.0-rc1-debug+ #1
Workqueue: tmr-fileio target_tmr_work [target_core_mod]
Call Trace:
 [<ffffffff810a6915>] lock_acquire+0x65/0x90
 [<ffffffff815e740b>] _raw_spin_lock_irqsave+0x4b/0x60
 [<ffffffffa03bc7ca>] target_release_cmd_kref+0x2a/0xa0 [target_core_mod]
 [<ffffffffa03bd418>] target_put_sess_cmd+0x28/0x50 [target_core_mod]
 [<ffffffffa03bad50>] core_tmr_lun_reset+0x390/0x640 [target_core_mod]
 [<ffffffffa03bce50>] target_tmr_work+0x80/0xd0 [target_core_mod]
 [<ffffffff81070e6d>] process_one_work+0x19d/0x430
 [<ffffffff8107120f>] worker_thread+0x10f/0x460
 [<ffffffff810772ba>] kthread+0xea/0x100
 [<ffffffff815e7a2f>] ret_from_fork+0x3f/0x70

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>