neighbour: annotate lockless accesses to n->nud_state

[ Upstream commit b071af52 ]

We have many lockless accesses to n->nud_state.

Before adding another one in the following patch,
add annotations to readers and writers.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 5baa0433 ("neighbour: fix data-races around n->output")
Signed-off-by: Sasha Levin <sashal@kernel.org>