xfs: enable in-core block reservation for rt metadata

In preparation for adding reverse mapping and refcounting to the
realtime device, enhance the metadir code to reserve free space for
btree shape changes as delayed allocation blocks.

This effectively allows us to pre-allocate space for the rmap and
refcount btrees in the same manner as we do for the data device
counterparts, which is how we avoid ENOSPC failures when space is low
but we've already committed to a COW operation.
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