l2tp_dfs_seq_start had a BUG_ON to catch a possible programming error in
l2tp_dfs_seq_open.

Since we can easily bail out of l2tp_dfs_seq_start, prefer to do that
and flag the error with a WARN_ON rather than crashing the kernel.
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>