memcg: sanitize __mem_cgroup_try_charge() call protocol

Some callsites pass a memcg directly, some callsites pass an mm that
then has to be translated to a memcg.  This makes for a terrible
function interface.

Just push the mm-to-memcg translation into the respective callsites and
always pass a memcg to mem_cgroup_try_charge().

[mhocko@suse.cz: add charge mm helper]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>