iplink: check for message truncation in iplink_get()

iplink: check for message truncation in iplink_get()

If message length exceeds maxlen argument of rtnl_talk(), it is truncated
to maxlen but unlike in the case of truncation to the length of local
buffer in rtnl_talk(), the caller doesn't get any indication of a problem.

In particular, iplink_get() passes the truncated message on and parsing it
results in various warnings and sometimes even a segfault (observed with
"ip link show dev ..." for a NIC with 125 VFs).

Handle message truncation in iplink_get() the same way as truncation in
rtnl_talk() would be handled: return an error.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>