target: Invoke release_cmd() callback without holding a spinlock

This patch fixes the following kernel warning because it avoids that
IRQs are disabled while ft_release_cmd() is invoked (fc_seq_set_resp()
invokes spin_unlock_bh()):

WARNING: CPU: 3 PID: 117 at kernel/softirq.c:150 __local_bh_enable_ip+0xaa/0x110()
Call Trace:
 [<ffffffff814f71eb>] dump_stack+0x4f/0x7b
 [<ffffffff8105e56a>] warn_slowpath_common+0x8a/0xc0
 [<ffffffff8105e65a>] warn_slowpath_null+0x1a/0x20
 [<ffffffff81062b2a>] __local_bh_enable_ip+0xaa/0x110
 [<ffffffff814ff229>] _raw_spin_unlock_bh+0x39/0x40
 [<ffffffffa03a7f94>] fc_seq_set_resp+0xe4/0x100 [libfc]
 [<ffffffffa02e604a>] ft_free_cmd+0x4a/0x90 [tcm_fc]
 [<ffffffffa02e6972>] ft_release_cmd+0x12/0x20 [tcm_fc]
 [<ffffffffa042bd66>] target_release_cmd_kref+0x56/0x90 [target_core_mod]
 [<ffffffffa042caf0>] target_put_sess_cmd+0xc0/0x110 [target_core_mod]
 [<ffffffffa042cb81>] transport_release_cmd+0x41/0x70 [target_core_mod]
 [<ffffffffa042d975>] transport_generic_free_cmd+0x35/0x420 [target_core_mod]

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Acked-by: Joern Engel <joern@logfs.org>
Reviewed-by: Andy Grover <agrover@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>