neighbour: Remove redundant assignment to err

'err' has been checked against 0 in the if statement.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250624014216.3686659-1-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>