Revert "net: net_namespace: Optimize the code"

This reverts commit 37fdbeb0 which is
commit 41467d2f 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: If449b818cb2b6a551d723ab75281f449e3ea78b7
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>