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>

tests/qemu-iotests/tests/mirror-sparse

@@ -96,13 +96,15 @@ _send_qemu_cmd $h1 '{"execute": "blockdev-del", "arguments":
                 {"node-name": "dst"}}' 'return' \
                   | _filter_block_job_offset | _filter_block_job_len
 $QEMU_IMG compare -U -f $IMGFMT -F raw $TEST_IMG.base $TEST_IMG
+# Some filesystems can fudge allocations for various reasons; rather
+# than expecting precise 2M and 20M images, it is better to allow for slop.
 result=$(disk_usage $TEST_IMG)
-if test $result -lt $((3*1024*1024)); then
+if test $result -lt $((4*1024*1024)); then
     actual=sparse
-elif test $result = $((20*1024*1024)); then
+elif test $result -gt $((19*1024*1024)); then
     actual=full
 else
-    actual=unknown
+    actual="unexpected size ($result)"
 fi
 echo "Destination is $actual; expected $expected"
 }