Right now the LSM labels for socketpairs are always uninitialized,
since there is no security hook for the socketpair() syscall. This
patch adds the required hooks so LSMs can properly label socketpairs.
This allows SO_PEERSEC to return useful information on those sockets.

Note that the behavior of socketpair() can be emulated by creating a
listener socket, connecting to it, and then discarding the initial
listener socket. With this workaround, SO_PEERSEC would return the
caller's security context. However, with socketpair(), the uninitialized
context is returned unconditionally. This is unexpected and makes
socketpair() less useful in situations where the security context is
crucial to the application.

With the new socketpair-hook this disparity can be solved by making
socketpair() return the expected security context.

Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: James Morris <james.morris@microsoft.com>