xfs: btree bulk loading

This series creates a bulk loading function for metadata btree cursors.

We start by creating the idea of a "fake root" for the btree type so
that we can use a special btree cursor to stage a new btree without
altering anything that might already exist.

Next, we add utility functions to compute the desired btree shape for a
given number of records.

Finally we extend all four per-AG btree cursor types to support staging
cursors and therefore bulk loading.  This will be used by upcoming patch
series to implement online repair and refactor offline repair.