netlink: add a nla_nest_end_safe() helper

netlink: add a nla_nest_end_safe() helper

The nla_len field in struct nlattr is a __u16, which can only hold
values up to 65535. If a nested attribute grows beyond this limit,
nla_nest_end() silently truncates the length, producing a corrupted
netlink message with no indication of the problem.

Since nla_nest_end() is used everywhere and this issue rarely happens,
let's add a new helper to check the length.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20260408-b4-ynl_ethtool-v2-4-7623a5e8f70b@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>