mISDN didn't define af_family_key_strings et al.

[Eric Paris <eparis@redhat.com>]

I see that recently AF_ISDN was added to the kernel but I don't see them
having updated af_family_key_strings[] found in net/core/sock.c.  I'm
adding a new one myself and caught a NULL entry toward the end of the
array.  My assumption is that isdn is using the sk_lock-AF_MAX entry
instead of their own.

As an aside it was nice having such a clean explosion when I didn't add
my own.  Are the AF_MAX entries in those arrays really needed?  If we
dropped it altogether would others (like myself and the isdn people) who
forgot to add their own get nice explosions with no other problems?

-Eric