memcg_write_event_control(): fix a user-triggerable oops

we are *not* guaranteed that anything past the terminating NUL
is mapped (let alone initialized with anything sane).

Fixes: 0dea1168 ("cgroup: implement eventfd-based generic API for notifications")
Cc: stable@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>