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

This reverts commit 3267b254 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: I4811e911553091a47d11e0b4ece358363809166c
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>