iotests: Improve mirror-sparse on ext4 and xfs

iotests: Improve mirror-sparse on ext4 and xfs

Fiona reported that an ext4 filesystem on top of LVM can sometimes
report over-allocation to du (based on the heuristics the filesystem
is making while observing the contents being mirrored); even though
the contents and actual size matched, about 50% of the time the size
reported by disk_usage was too large by 4k, failing the test.  In
auditing other iotests, this is a common problem we've had to deal
with.

Meanwhile, Markus reported that an xfs filesystem reports disk usage
at a default granularity of 1M (so the sparse file occupies 3M, since
it has just over 2M data).

Reported-by: Fiona Ebner <f.ebner@proxmox.com>
Reported-by: Markus Armbruster <armbru@redhat.com>
Fixes: c0ddcb2c ("tests: Add iotest mirror-sparse for recent patches")
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250523163041.2548675-7-eblake@redhat.com>
[eblake: Also fix xfs issue]
Signed-off-by: Eric Blake <eblake@redhat.com>