[DECnet]: Patch to fix recvmsg() flag check

This patch means that 64bit kernel/32bit userland platforms will now
work correctly with DECnet.

Signed-off-by: Patrick Caulfield <patrick@tykepenguin.com>
Signed-off-by: Steven Whitehouse <steve@chygwyn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>