xfstests: add a multithreaded mode to bstat

xfstests: add a multithreaded mode to bstat

For benchmarking of bulkstat, add a multithreaded mode that spawns a
thread per AG and runs bulkstat on every AG in parallel. There is a
small amount of overlap between each AG because of the way the
interface works only on inode numbers, so some inodes are reported
twice. A real implementation of this sort of parallelism would be
greatly helped by adding an AG parameter to the bulkstat interface.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>