libnetlink: Double the dump buffer size

libnetlink: Double the dump buffer size

There have been reports about 'ip addr' printing "Message truncated" on
systems with large numbers of VFs. Although I haven't been able to get
my hands on hardware suitable to reproduce this, increasing the dump
buffer has been reported to resolve the issue. For want of a better
idea, just double the buffer size to 32k.

Feels like this opportunistic buffer size selection is rather
workarounding a design flaw in libnetlink or maybe even the netlink
protocol itself.

Signed-off-by: Phil Sutter <phil@nwl.cc>