net: inet: diag: expose the socket mark to privileged processes.

This adds the capability for a process that has CAP_NET_ADMIN on
a socket to see the socket mark in socket dumps.

Commit a52e95abf7 ("net: diag: allow socket bytecode filters to
match socket marks") recently gave privileged processes the
ability to filter socket dumps based on mark. This patch is
complementary: it ensures that the mark is also passed to
userspace in the socket's netlink attributes.  It is useful for
tools like ss which display information about sockets.

Tested: https://android-review.googlesource.com/270210
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/inet_diag.h

@@ -123,6 +123,7 @@ enum {
 	INET_DIAG_LOCALS,
 	INET_DIAG_PEERS,
 	INET_DIAG_PAD,
+	INET_DIAG_MARK,
 	__INET_DIAG_MAX,
 };