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.
-----BEGIN PGP SIGNATURE-----
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=Flk0
-----END PGP SIGNATURE-----