xfs: repair the rmapbt

Rebuild the reverse mapping btree from all primary metadata.  This first
patch establishes the bare mechanics of finding records and putting
together a new ondisk tree; more complex pieces are needed to make it
work properly.

Link: https://docs.kernel.org/filesystems/xfs-online-fsck-design.html#case-study-rebuilding-reverse-mapping-records
Link: https://docs.kernel.org/filesystems/xfs-online-fsck-design.html#case-study-reaping-after-repairing-reverse-mapping-btrees
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
6 files changed