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

This reverts commit c261dcd6 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: I6712463b3c2f28cab861132817ad6f8f7817b74e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>