neighbour: Annotate access to struct pneigh_entry.{flags,protocol}.

neighbour: Annotate access to struct pneigh_entry.{flags,protocol}.

We will convert pneigh readers to RCU, and its flags and protocol
will be read locklessly.

Let's annotate the access to the two fields.

Note that all access to pn->permanent is under RTNL (neigh_add()
and pneigh_ifdown_and_unlock()), so WRITE_ONCE() and READ_ONCE()
are not needed.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250716221221.442239-9-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>