netlink: add const variants of NLA iteration macros

Add const-qualified versions of the NLA iteration helpers to allow
walking immutable netlink attribute buffers without discarding const
qualifiers.

This introduces NLA_NEXT_CONST(), _NLA_END_CONST(), and
NLA_FOREACH_CONST() in netlink_snl.h.

Signed-off-by: Ishan Agrawal <iagrawal9990@gmail.com>
Sponsored-by : Google LLC (GSoC 2026)