netlink: specs: rt-neigh: prefix struct nfmsg members with ndm

netlink: specs: rt-neigh: prefix struct nfmsg members with ndm

Attach ndm- to all members of struct nfmsg. We could possibly
use name-prefix just for C, but I don't think we have any precedent
for using name-prefix on structs, and other rtnetlink sub-specs
give full names for fixed header struct members.

Fixes: bc515ed06652 ("netlink: specs: Add a spec for neighbor tables in rtnetlink")
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250414211851.602096-9-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>