Merge branch 'l2tp-races'

Cong Wang says:

====================
l2tp: fix race conditions in l2tp_tunnel_register()

This patchset contains two patches, the first one is a preparation for
the second one which is the actual fix. Please find more details in
each patch description.

I have ran the l2tp test (https://github.com/katalix/l2tp-ktest),
all test cases are passed.

v3: preserve EEXIST errno for user-space
v2: move IDR allocation to l2tp_tunnel_register()
====================

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