[DECNET]: Tidy send side socket SKB allocation.

Patch from Steve Whitehouse which I've vetted and tested:

"This patch is really intended has a move towards fixing the
sendmsg/recvmsg functions in various ways so that we will finally
have working nagle. Also reduces code duplication."

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