[SCSI] libfc: Remove the reference to FCP packet from scsi_cmnd in case of error

fc_queuecommand() allocates an FCP packet for each SCSI command and sends
it out on the wire. In the process it stores the reference to the FCP packet
in the scsi_cmnd structure.
Now, in case under stress testing the libfc exchange layer runs out of
exchanges the fc_queuecommand() may not be able to send out commands out on
the wire. In such a scenario if there is an error in sending the FCP packet
out the wire; fc_queuecommand() deletes the FCP packet from internal queue,
releases the FCP packet and returns a SCSI_MLQUEUE_HOST_BUSY status to the
scsi-ml. But, the reference to the FCP packet set in the scsi_cmnd is not
removed from the scsi_cmnd in this code path.

This might lead to a crash under stress testing where the scsi_cmnd failed by
fc_queuecommand() comes up to fc_eh_abort() via scsi eh thread. fc_eh_abort()
will get reference to the FCP packet to be aborted from the scsi_cmnd for
further FCP abort related processing and then try to release the FCP packet
that has already been released.

This patch removes the FCP packet reference from the scsi_cmnd before returning
back from fc_queuecommand() in case of an error in sending out the FCP packet.

Signed-off-by: Neerav Parikh <Neerav.Parikh@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>