Smack: check for 'struct socket' with NULL sk

There's a small problem with smack and NFS. A similar report was also
sent here: http://lkml.org/lkml/2007/10/27/85

I've also added similar checks in inode_{get/set}security().  Cheating from
SELinux post_create_socket(), it does the same.

[akpm@linux-foundation.org: remove uneeded BUG_ON()]
Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Acked-by: Casey Schaufler <casey@schuafler-ca.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>