tcm_fc: Check for aborted sequence

Add a check for an aborted sequence, which has a
NULL sequence pointer, to avoid target crashes.
The most relevant messages from the crash (entered
from video capture) include:

BUG: unable to handle kernel paging request at ffffffffffffffdf
IP: [<ffffffffa02d514c>] fc_seq_send+0x3c/0x150 [libfc]
...
Call Trace:
 [<ffffffffa0443de6>] ft_queue_data_in+0x266/0x560 [tcm_fc]
Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/tcm_fc/tfc_io.c

@@ -103,6 +103,13 @@ int ft_queue_data_in(struct se_cmd *se_cmd)
 	use_sg = !(remaining % 4);
 
 	while (remaining) {
+		struct fc_seq *seq = cmd->seq;
+
+		if (!seq) {
+			pr_debug("%s: Command aborted, xid 0x%x\n",
+				 __func__, ep->xid);
+			break;
+		}
 		if (!mem_len) {
 			sg = sg_next(sg);
 			mem_len = min((size_t)sg->length, remaining);
@@ -169,7 +176,7 @@ int ft_queue_data_in(struct se_cmd *se_cmd)
 			f_ctl |= FC_FC_END_SEQ;
 		fc_fill_fc_hdr(fp, FC_RCTL_DD_SOL_DATA, ep->did, ep->sid,
 			       FC_TYPE_FCP, f_ctl, fh_off);
-		error = lport->tt.seq_send(lport, cmd->seq, fp);
+		error = lport->tt.seq_send(lport, seq, fp);
 		if (error) {
 			/* XXX For now, initiator will retry */
 			pr_err_ratelimited("%s: Failed to send frame %p, "