l2tp: remove BUG_ON in l2tp_tunnel_closeall

l2tp_tunnel_closeall is only called from l2tp_core.c, and it's easy
to statically analyse the code path calling it to validate that it
should never be passed a NULL tunnel pointer.

Having a BUG_ON checking the tunnel pointer triggers a checkpatch
warning.  Since the BUG_ON is of no value, remove it to avoid the
warning.

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>