cgroup: memcg: pass correct argument to subsys_cgroup_allow_attach

Pass correct argument to subsys_cgroup_allow_attach(), which
expects 'struct cgroup_subsys_state *' argument but we pass
'struct cgroup *' instead which doesn't seem right.

This fixes following 'incompatible pointer type' compiler warning:
----------
  CC      mm/memcontrol.o
mm/memcontrol.c: In function ‘mem_cgroup_allow_attach’:
mm/memcontrol.c:5052:2: warning: passing argument 1 of ‘subsys_cgroup_allow_attach’ from incompatible pointer type [enabled by default]
In file included from include/linux/memcontrol.h:22:0,
                 from mm/memcontrol.c:29:
include/linux/cgroup.h:953:5: note: expected ‘struct cgroup_subsys_state *’ but argument is of type ‘struct cgroup *’
----------

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>