scsi: iscsi_tcp: restrict to TCP sockets

scsi: iscsi_tcp: restrict to TCP sockets

Nothing prevents iscsi_sw_tcp_conn_bind() to receive file descriptor
pointing to non TCP socket (af_unix for example).

Return -EINVAL if this is attempted, instead of crashing the kernel.

Fixes: 7ba247138907 ("[SCSI] open-iscsi/linux-iscsi-5 Initiator: Initiator code")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Lee Duncan <lduncan@suse.com>
Cc: Chris Leech <cleech@redhat.com>
Cc: Mike Christie <michael.christie@oracle.com>
Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: open-iscsi@googlegroups.com
Cc: linux-scsi@vger.kernel.org
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>