[SCSI] iscsi_tcp: hook iscsi_tcp into iscsi_endpoint code

iscsi_tcp creates its ep in userspace using sockets because
it is virtual, so we just check if we are sent a ep and fail
if we are.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>