iotests: Filter 175's allocation information

iotests: Filter 175's allocation information

It is possible for an empty file to take up blocks on a filesystem, for
example:

$ qemu-img create -f raw test.img 1G
Formatting 'test.img', fmt=raw size=1073741824
$ mkfs.ext4 -I 128 -q test.img
$ mkdir test-mount
$ sudo mount -o loop test.img test-mount
$ sudo touch test-mount/test-file
$ stat -c 'blocks=%b' test-mount/test-file
blocks=8

These extra blocks (one cluster) are apparently used for metadata,
because they are always there, on top of blocks used for data:

$ sudo dd if=/dev/zero of=test-mount/test-file bs=1M count=1
1+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00135339 s, 775 MB/s
$ stat -c 'blocks=%b' test-mount/test-file
blocks=2056

Make iotest 175 take this into account.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Message-id: 20190516144319.12570-1-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>