[X25]: fix for spinlock recurse and spinlock lockup with timer handler

When the sk_timer function x25_heartbeat_expiry() is called by the
kernel in a running/terminating process, spinlock-recursion and
spinlock-lockup locks up the kernel.  This has happened with testing
on some distro's and the patch below fixed it.

Signed-off-by: Shaun Pereira <spereira@tusc.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>