xfs: support dynamic btree cursor height

In this series, we rearrange the incore btree cursor so that we can
support btrees of any height.  This will become necessary for realtime
rmap and reflink since we'd like to handle tall trees without bloating
the AG btree cursors.

Chandan Babu pointed out that his large extent counters series depends
on the ability to have btree cursors of arbitrary heights, so I've
ported this to 5.15-rc1 so his patchsets won't have to depend on
djwong-dev for submission.

v2: reduce scrub btree checker memory footprint even more, put the one
    fixpatch first, use struct_size, fix 80col problems, move all the
    btree zone work to a separate series
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