networkd: rearrange checks when to write something into sysctl a bit

networkd: rearrange checks when to write something into sysctl a bit

Move check whether ipv6 is available into link_ipv6_privacy_extensions()
to keep it as internal and early as possible.

Always check if there's a network attached to a link before we apply
sysctls. We do this for most of the sysctl functions already, with this
change we do it for all.