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

This reverts commit dac46598 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: Ia50d530165546066c7847e0a48a68176269081fa
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>