neighbour: switch to standard rcu, instead of rcu_bh

[ Upstream commit 09eed119 ]

rcu_bh is no longer a win, especially for objects freed
with standard call_rcu().

Switch neighbour code to no longer disable BH when not necessary.

Signed-off-by: Eric Dumazet <edumazet@google.com>
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>