[NetLabel]: change the SELinux permissions

Change NetLabel to use the 'recvfrom' socket permission and the
SECINITSID_NETMSG SELinux SID as the NetLabel base SID for incoming packets.
This patch effectively makes the old, and currently unused, SELinux NETMSG
permissions NetLabel permissions.

Signed-of-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>