xfs_repair: create a new class of btree rebuild cursors

xfs_repair: create a new class of btree rebuild cursors

Create some new support structures and functions to assist phase5 in
using the btree bulk loader to reconstruct metadata btrees.  This is the
first step in removing the open-coded AG btree rebuilding code.

Note: The code in this patch will not be used anywhere until the next
patch, so warnings about unused symbols are expected.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
[for all but the slack_node change from 0 to 2:]
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>