memcontrol: implement mem_cgroup_tryget_online()

This patch implements a helper function that try to get a reference to an
memcg's css, as well as checking if it is online.  This new function is
almost exactly the same as the existing mem_cgroup_tryget(), except for
the onlineness check.  In the !CONFIG_MEMCG case, it always returns true,
analogous to mem_cgroup_tryget().  This is useful for e.g to the new zswap
writeback scheme, where we need to select the next online memcg as a
candidate for the global limit reclaim.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Nhat Pham <[email protected]>
Tested-by: Bagas Sanjaya <[email protected]>
Reviewed-by: Yosry Ahmed <[email protected]>
Cc: Chris Li <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Domenico Cerasuolo <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Vitaly Wool <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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