neighbour: delete redundant judgment statements

[ Upstream commit c25bdd2ac8cf7da70a226f1a66cdce7af15ff86f ]

The initial value of err is -ENOBUFS, and err is guaranteed to be
less than 0 before all goto errout. Therefore, on the error path
of errout, there is no need to repeatedly judge that err is less than 0,
and delete redundant judgments to make the code more concise.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: becbd5850c03 ("neighbour: use RCU protection in __neigh_notify()")
Signed-off-by: Sasha Levin <sashal@kernel.org>