[SCSI] fcoe: fix regression on offload em matching function for initiator/target

This is a regression introduced by commit
1ff9918b625457ce20d450d00f9ed0a12ba191b7 The else statement here is breaking
the initiator logic of allocating xid from the offloaded em xid pool for READ
I/O only to use DDP, as shown by the snippet of trace below, where the WRITE
is using xid 0x5 from the offloaded em xid pool:

Protocol VID Len S_ID        D_ID     OX_ID  RX_ID  Summary
..
*FCP     228 96  0b.08.01 -> 01.0f.00 0x0005 0xffff SCSI: Write(10) LUN: 0x00
 FCP     228 76  01.0f.00 -> 0b.08.01 0x0005 0x828d XFER_RDY
...

The bug is in the else statement, for both initiator and target, the
new command will have FC frame header bit 23 (FC_FC_EX_CTX) cleared as it was
originated from the initiator. Also, this is assuming the frame header is
already filled up, which is only true for target since for initiator, this is a
new frame and oem_match gets called when em tries get xid for this i/o before
it is filled up and sent out.

The fix is to check if there is a fc_fcp_pkt associated w/ this frame from
fr_fsp(fp), since fr_fsp(fp) is NULL for tcm_fc target and non-I/O frame in
initiator. This should also return true for target only if it is an
FC_RCTL_DD_UNSOL_CMD and rx_id is not allocated.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/fcoe/fcoe.c

@@ -756,11 +756,12 @@ bool fcoe_oem_match(struct fc_frame *fp)
 	if (fc_fcp_is_read(fr_fsp(fp)) &&
 	    (fr_fsp(fp)->data_len > fcoe_ddp_min))
 		return true;
-	else if (!(ntoh24(fh->fh_f_ctl) & FC_FC_EX_CTX)) {
+	else if ((fr_fsp(fp) == NULL) &&
+		 (fh->fh_r_ctl == FC_RCTL_DD_UNSOL_CMD) &&
+		 (ntohs(fh->fh_rx_id) == FC_XID_UNKNOWN)) {
 		fcp = fc_frame_payload_get(fp, sizeof(*fcp));
-		if (ntohs(fh->fh_rx_id) == FC_XID_UNKNOWN &&
-		    fcp && (ntohl(fcp->fc_dl) > fcoe_ddp_min) &&
-		    (fcp->fc_flags & FCP_CFL_WRDATA))
+		if ((fcp->fc_flags & FCP_CFL_WRDATA) &&
+		    (ntohl(fcp->fc_dl) > fcoe_ddp_min))
 			return true;
 	}
 	return false;