l2tp: fix tunnel and session refcount leak on seq_file release

In the Linux kernel, the following vulnerability has been resolved:

l2tp: fix tunnel and session refcount leak on seq_file release

In pppol2tp_proc_open() and l2tp_dfs_seq_open(), iteration state
(pd->tunnel and pd->session) is kept in seq_file private data to allow
iteration across multiple read() system calls.

However, if userspace closes /proc/net/pppol2tp or /sys/kernel/debug/l2tp/tunnels
before reading to end-of-file (EOF), any tunnel or session reference stored in
pd->tunnel / pd->session is left un-dropped when seq_file private data is freed.

Fix this by dropping any remaining pd->tunnel and pd->session references in
pppol2tp_proc_release() and l2tp_dfs_seq_release() when closing the file.