Revert "net: defer final 'struct net' free in netns dismantle"

This reverts commit b7a79e51 which is
commit 0f6ede9f upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I5ce12f6c29c79bcbcf82ff8ce3f7edc63ff694f0
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>