iser-target: Fix NULL dereference in SW mode DIF

Fallback to software mode DIF if HCA does not support
PI (without crashing obviously). It is still possible to
run with backend protection and an unprotected frontend,
so looking at the command prot_op is not enough. Check
device PI capability on a per-IO basis (isert_prot_cmd
inline static) to determine if we need to handle protection
information.

Trace:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: [<ffffffffa037f8b1>] isert_reg_sig_mr+0x351/0x3b0 [ib_isert]
Call Trace:
 [<ffffffff812b003a>] ? swiotlb_map_sg_attrs+0x7a/0x130
 [<ffffffffa038184d>] isert_reg_rdma+0x2fd/0x370 [ib_isert]
 [<ffffffff8108f2ec>] ? idle_balance+0x6c/0x2c0
 [<ffffffffa0382b68>] isert_put_datain+0x68/0x210 [ib_isert]
 [<ffffffffa02acf5b>] lio_queue_data_in+0x2b/0x30 [iscsi_target_mod]
 [<ffffffffa02306eb>] target_complete_ok_work+0x21b/0x310 [target_core_mod]
 [<ffffffff8106ece2>] process_one_work+0x182/0x3b0
 [<ffffffff8106fda0>] worker_thread+0x120/0x3c0
 [<ffffffff8106fc80>] ? maybe_create_worker+0x190/0x190
 [<ffffffff8107594e>] kthread+0xce/0xf0
 [<ffffffff81075880>] ? kthread_freezable_should_stop+0x70/0x70
 [<ffffffff8159a22c>] ret_from_fork+0x7c/0xb0
 [<ffffffff81075880>] ? kthread_freezable_should_stop+0x70/0x70

Reported-by: Slava Shwartsman <valyushash@gmail.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Cc: <stable@vger.kernel.org> # v3.14+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>