mm: memcg/slab: fix obj_cgroup_charge() return value handling

Commit 10befea9 ("mm: memcg/slab: use a single set of kmem_caches
for all allocations") introduced a regression into the handling of the
obj_cgroup_charge() return value.  If a non-zero value is returned
(indicating of exceeding one of memory.max limits), the allocation
should fail, instead of falling back to non-accounted mode.

To make the code more readable, move memcg_slab_pre_alloc_hook() and
memcg_slab_post_alloc_hook() calling conditions into bodies of these
hooks.

Fixes: 10befea9 ("mm: memcg/slab: use a single set of kmem_caches for all allocations")
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20201127161828.GD840171@carbon.dhcp.thefacebook.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>