openvswitch: fix error return code in validate_and_copy_dec_ttl()

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

Changing 'return start' to 'return action_start' can fix this bug.

Fixes: 69929d4c

 ("net: openvswitch: fix TTL decrement action netlink message format")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Reviewed-by: Eelco Chaudron <echaudro@redhat.com>
Link: https://lore.kernel.org/r/20201204114314.1596-1-wanghai38@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>