xfs: fix cpu hotplug mess [v2]

Ritesh and Eric separately reported crashes in XFS's hook function for
CPU hot remove if the remove event races with a filesystem being
mounted.  I also noticed via generic/650 that once in a while the log
will shut down over an apparent overrun of a transaction reservation;
this turned out to be due to CIL percpu list aggregation failing to pick
up the percpu list items from a dying CPU.

Either way, the solution here is to eliminate the need for a CPU dying
hook by using a private cpumask to track which CPUs have added to their
percpu lists directly, and iterating with that mask.  This fixes the log
problems and (I think) solves a theoretical UAF bug in the inodegc code
too.

v2: fix a few put_cpu uses, add necessary memory barriers, and use
    atomic cpumask operations

This has been lightly tested with fstests.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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