net/appletalk: fix atalk_release use after free

The BKL removal in appletalk introduced a use-after-free problem,
where atalk_destroy_socket frees a sock, but we still release
the socket lock on it.

An easy fix is to take an extra reference on the sock and sock_put
it when returning from atalk_release.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>