genetlink: fix counting regression on ctrl_dumpfamily()

Commit 2ae0f17d ("genetlink: use idr to track families") replaced

	if (++n < fams_to_skip)
		continue;
into:

	if (n++ < fams_to_skip)
		continue;

This subtle change cause that on retry ctrl_dumpfamily() call we omit
one family that failed to do ctrl_fill_info() on previous call, because
cb->args[0] = n number counts also family that failed to do
ctrl_fill_info().

Patch fixes the problem and avoid confusion in the future just decrease
n counter when ctrl_fill_info() fail.

User visible problem caused by this bug is failure to get access to
some genetlink family i.e. nl80211. However problem is reproducible
only if number of registered genetlink families is big enough to
cause second call of ctrl_dumpfamily().

Cc: Xose Vazquez Perez <xose.vazquez@gmail.com>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Johannes Berg <johannes@sipsolutions.net>
Fixes: 2ae0f17d ("genetlink: use idr to track families")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>