Fix: error logged on partial recvmsg() in MSG_DONTWAIT

The relay daemon logs a "Resource temporarily unavailable" error
message when the lttcomm_recvmsg_inet_sock() is invoked and
no data is left to be consumed from the lttcomm_sock.

The "recvmsg" socket operation is called in a loop by the relay
daemon to consume the data being received in 64k chunks. If, on
one of those iterations, 0 bytes are available, recvmsg() will
return an error (-1, errno = EAGAIN). This should not be
logged in non-blocking mode.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>