libnetlink: parse_rtattr_nested should allow NLA_F_NESTED flag

libnetlink: parse_rtattr_nested should allow NLA_F_NESTED flag

In kernel commit 8cb081746c03 ("netlink: make validation more configurable
for future strictness"), Linux started implicitly flagging nests with
NLA_F_NESTED, unless the nest is created with nla_nest_start_noflag().

The ETS code uses nla_nest_start() where possible, so it does not work with
the current iproute2 code. Have libnetlink catch up by admitting the flag
in the attribute.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>