af_unix: Allow SO_PEERCRED to work across namespaces.

af_unix: Allow SO_PEERCRED to work across namespaces.

Use struct pid and struct cred to store the peer credentials on struct
sock.  This gives enough information to convert the peer credential
information to a value relative to whatever namespace the socket is in
at the time.

This removes nasty surprises when using SO_PEERCRED on socket
connetions where the processes on either side are in different pid and
user namespaces.

Signed-off-by: Eric W. Biederman <[email protected]>
Acked-by: Daniel Lezcano <[email protected]>
Acked-by: Pavel Emelyanov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>