vxlan: fix error return code in __vxlan_dev_create()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 0ce1822c ("vxlan: add adjacent link to limit depth level")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Link: https://lore.kernel.org/r/1606903122-2098-1-git-send-email-zhangchangzhong@huawei.com


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