taskstats: cgroupstats_user_cmd() may leak on error

If prepare_reply() succeeds we have allocated memory for 'rep_skb'.  If
nla_reserve() then subsequently fails and returns NULL we fail to release
the memory we allocated, thus causing a leak.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Cc: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>