Revert "rtnetlink: Reject negative ifindexes in RTM_NEWLINK"

This reverts commit 42c84064 which is
commit 30188bd7838c16a98a520db1fe9df01ffc6ed368 upstream.

It was improperly backported to 4.19.y, and applied to the wrong
function, which obviously causes problems.  A fixed version will be
applied as a separate commit later.

Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/ZSQeA8fhUT++iZvz@ostr-mac


Cc: Ido Schimmel <idosch@nvidia.com>
Cc: Jiri Pirko <jiri@nvidia.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>