net: warn about attempts to register negative ifindex

Since the xarray changes we mix returning valid ifindex and negative
errno in a single int returned from dev_index_reserve(). This depends
on the fact that ifindexes can't be negative. Otherwise we may insert
into the xarray and return a very large negative value. This in turn
may break ERR_PTR().

OvS is susceptible to this problem and lacking validation (fix posted
separately for net).

Reject negative ifindex explicitly. Add a warning because the input
validation is better handled by the caller.

Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230814205627.2914583-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>