Fix computation of SKB size for quota messages

Fix computation of size of skb needed for quota message.  We should use
netlink provided functions and not just an ad-hoc number.  Also don't print
the return value from nla_put_foo() as it is always -1.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>