device: negate the condition in `_get_maybe_ipv6_disabled`
Summary
device: negate the condition in _get_maybe_ipv6_disabled
Purpose
The condition in `_get_maybe_ipv6_disabled()` is improperly set which
returns the wrong value on if an device is disabled or not when
generating the assume connection. And when
`/proc/sys/net/ipv6/conf/$DEV/disable_ipv6` is not existed (not
disabling ipv6 through sysctl setting), IPv6 is disabled by default.
Checklist
Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:
-
the subject for all commits is concise and explicative -
the message for all commits explains the reason for the change -
the source is properly formatted -
any relevant documentation is up to date -
you have added unit tests if applicable