device: add IPv6 sysfs existence check in some ipv6 sysctl functions

Summary

device: add IPv6 sysfs existence check in _set_state_full()

Purpose

When the kernel boot parameter ipv6.disable=1 is set, NetworkManager attempts to read files under /proc/sys/net/ipv6, resulting in numerous error messages in the debug logs. For example:

# journalctl -u NetworkManager | grep error | grep ipv6
NetworkManager[758]: <debug> [1726699000.9384] platform-linux: error reading /proc/sys/net/ipv6/conf/lo/disable_ipv6: Failed to open file "/proc/sys/net/ipv6/conf/lo/disable_ipv6": No such file or directory
NetworkManager[758]: <debug> [1726699000.9400] platform-linux: error reading /proc/sys/net/ipv6/conf/lo/accept_ra: Failed to open file "/proc/sys/net/ipv6/conf/lo/accept_ra": No such file or directory
NetworkManager[758]: <debug> [1726699000.9401] platform-linux: error reading /proc/sys/net/ipv6/conf/lo/disable_ipv6: Failed to open file "/proc/sys/net/ipv6/conf/lo/disable_ipv6": No such file or directory
NetworkManager[758]: <debug> [1726699000.9401] platform-linux: error reading /proc/sys/net/ipv6/conf/lo/hop_limit: Failed to open file "/proc/sys/net/ipv6/conf/lo/hop_limit": No such file or directory
NetworkManager[758]: <debug> [1726699000.9401] platform-linux: error reading /proc/sys/net/ipv6/conf/lo/use_tempaddr: Failed to open file "/proc/sys/net/ipv6/conf/lo/use_tempaddr": No such file or directory
NetworkManager[758]: <debug> [1726699000.9401] platform-linux: error reading /proc/sys/net/ipv6/conf/lo/temp_valid_lft: Failed to open file "/proc/sys/net/ipv6/conf/lo/temp_valid_lft": No such file or directory
NetworkManager[758]: <debug> [1726699000.9401] platform-linux: error reading /proc/sys/net/ipv6/conf/lo/temp_prefered_lft: Failed to open file "/proc/sys/net/ipv6/conf/lo/temp_prefered_lft": No such file or directory
...

This also results unnecessary system calls by attempting to open non-existent sysfs.
This patch adds checks in some ipv6 sysctl functions to verify the existence of /proc/sys/net/ipv6.

There are still other paths that attempts to open IPv6 sysfs so it won't eliminate all reading errors.
However, in my environment, the number of errors was reduced from 20 to 2 and I suppose this is effective.

Before the patch applied:

# journalctl -u NetworkManager | grep error | grep ipv6 | wc -l
20

After the patche applied:

# journalctl -u NetworkManager | grep error | grep ipv6 | wc -l
2