cgroup: Fix build failure when CONFIG_SHRINKER_DEBUG

fa7e439c ("cgroup: Homogenize cgroup_get_from_id() return value") broken
build when CONFIG_SHRINKER_DEBUG by trying to return an errno from
mem_cgroup_get_from_ino() which returns struct mem_cgroup *. Fix by using
ERR_CAST() instead.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Michal Koutný <mkoutny@suse.com>f
Fixes: fa7e439c ("cgroup: Homogenize cgroup_get_from_id() return value")

mm/memcontrol.c

@@ -5111,7 +5111,7 @@ struct mem_cgroup *mem_cgroup_get_from_ino(unsigned long ino)
 
 	cgrp = cgroup_get_from_id(ino);
 	if (IS_ERR(cgrp))
-		return PTR_ERR(cgrp);
+		return ERR_CAST(cgrp);
 
 	css = cgroup_get_e_css(cgrp, &memory_cgrp_subsys);
 	if (css)