netlink: reduce default log levels

Reduce the default log level for netlink to LOG_INFO. This removes a
number of messages such as

[nl_iface] dump_sa: unsupported family: 0, skipping

or

[nl_iface] get_operstate_ether: error calling SIOCGIFMEDIA on vlan0: 22

that are useful for debugging, but not for most users.

Reviewed by: melifaro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D40062