xfs: enable in-core block reservation for rt metadata

In preparation for adding reverse mapping and refcounting to the
realtime device, enhance the per-AG block reservation system for use
with realtime metadata.  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.