Merge branch 'l2tp-fix-some-races-in-session-deletion'

Guillaume Nault says:

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l2tp: fix some races in session deletion

L2TP provides several interfaces for deleting sessions. Using two of
them concurrently can lead to use-after-free bugs.

Patch #2 uses a flag to prevent double removal of L2TP sessions.
Patch #1 fixes a bug found in the way. Fixing this bug is also
necessary for patch #2 to handle all cases.

This issue is similar to the tunnel deletion bug being worked on by
Sabrina: https://patchwork.ozlabs.org/patch/814173/
====================

Signed-off-by: David S. Miller <davem@davemloft.net>