nlattr: add size check to decode_nla_xval and decode_nla_flags

nlattr: add size check to decode_nla_xval and decode_nla_flags

If the field size is known, do not print as valid numbers those
attributes that are too short, and ignore extra data.

This change puts xval/flags printing in line with the reset of attribute
decoders.

* nlattr.h (struct decode_nla_xlat_opts): Add size field.
* nlattr.c (decode_nla_xval, decode_nla_flags): Check that len is at
least opts->size, limit len to opts->size.
(decode_nla_ip_proto): Specify size of 1.

Co-Authored-by: Dmitry V. Levin <ldv@altlinux.org>