totemknet: Handle trncated packets and msg_flags

Code was taken from totemudpu but same cannot be really used for knet.

Checking of received number of bytes doesn't work as with UDP(U)
because knet might send KNET_MAX_PACKET_SIZE. It is also bug
if message is truncated because knet always sends packet with maximum
size of KNET_MAX_PACKET_SIZE.

It probably doesn't make too much sense to force having
msg_hdr.msg_flags, but it is also good to know platform
(or configuration) doesn't support them, so just issue
compiler warning.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>