xfs_repair: rebuild free space btrees with bulk loader

xfs_repair: rebuild free space btrees with bulk loader

Use the btree bulk loading functions to rebuild the free space btrees
and drop the open-coded implementation.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
[sandeen: move phase5_func changes into this patch]
[sandeen: adjust for prior inclusion of "fix rebuilding btree block..."]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>