xfs_scrub: scrub realtime allocation group metadata

xfs_scrub: scrub realtime allocation group metadata

Scan realtime group metadata as part of phase 2, just like we do for AG
metadata.  For pre-rtgroup filesystems, pretend that this is a "rtgroup
0" scrub request because the kernel expects that.  Replace the old
cond_wait code with a scrub barrier because they're equivalent for two
items that cannot be scrubbed in parallel.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>