[LLC]: allow datagram recvmsg

LLC receive is broken for SOCK_DGRAM.
If an application does recv() on a datagram socket and there
is no data present, don't return "not connected". Instead, just
do normal datagram semantics.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>