xfs: document full filesystem scans for online fsck

Certain parts of the online fsck code need to scan every file in the
entire filesystem.  It is not acceptable to block the entire filesystem
while this happens, which means that we need to be clever in allowing
scans to coordinate with ongoing filesystem updates.  We also need to
hook the filesystem so that regular updates propagate to the staging
records.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>