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 each of the btree
root types (AG header and inode) 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, load records into new leaf blocks, compute the
node blocks from that, and present the new root ready for commit.

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.

For v4, fix a lot of review comments from Brian Foster, most of which
relate to disentangling thornier parts of the code; and clarifying the
documentation so that someone other than the author can understand what
is going on here. :)

For v5, a few documentation tweaks and moving the btree staging code to
a separate file since it's really a new use for btree cursors.
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