taskstats: fix nl parsing in accounting/getdelays.c

The type TASKSTATS_TYPE_NULL should always be ignored.

When jumping to the next attribute, only the length of the current
attribute should be added, not the length of all nested attributes.
This last bug was not visible before commit 80df5542, because the
kernel didn't put more than two nested attributes.

Fixes: a3baf649 ("[PATCH] per-task-delay-accounting: documentation")
Fixes: 80df5542 ("taskstats: use the libnl API to align nlattr on 64-bit")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>