netlink plugin: Use of less strict rules in link_filter_cb()

netlink plugin: Use of less strict rules in link_filter_cb()

mnl_attr_validate2() function implements strict equality check of kernel and
userspace structures size. Additional counters was added to 4.6 Linux kernel,
sizes was changed and mismatch can occur.

This patch weakened validation.
Now Collectd just checks if structures, received from kernel space, has enough
data.

Closes: #2510

# Conflicts:
#	src/netlink.c