[DECNET]: fix SIGPIPE

Currently recvmsg generates SIGPIPE whereas sendmsg does not; for the
other stacks it seems to be the other way round!

It also fixes the bug where reading from a socket whose peer has shutdown
returned -EINVAL rather than 0.

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