xfs: add helpers to dispose of old btree blocks after a repair
Now that we've plumbed in the ability to construct a list of dead btree
blocks following a repair, add more helpers to dispose of them. This is
done by examining the rmapbt -- if the btree was the only owner we can
free the block, otherwise it's crosslinked and we can only remove the
rmapbt record.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Reviewed-by: Dave Chinner <dchinner@redhat.com>