Revert "net: rtnetlink: remove redundant assignment to variable err"

This reverts commit c49d3180 which is
commit 7d311801 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: I29e9d676be00412288460401a10b4c456aa90f99
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>