xfs: read leaf blocks when computing keys for bulkloading into node blocks

When constructing a new btree, xfs_btree_bload_node needs to read the
btree blocks for level N to compute the keyptrs for the blocks that will
be loaded into level N+1.  The level N blocks must be formatted at that
point.

A subsequent patch will change the btree bulkloader to write new btree
blocks in 256K chunks to moderate memory consumption if the new btree is
very large.  As a consequence of that, it's possible that the buffers
for lower level blocks might have been reclaimed by the time the node
builder comes back to the block.

Therefore, change xfs_btree_bload_node to read the lower level blocks
to handle the reclaimed buffer case.  As a side effect, the read will
increase the LRU refs, which will bias towards keeping new btree buffers
in memory after the new btree commits.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>