fiemap-tester: holes can be backed by unwritten extents

fiemap-tester: holes can be backed by unwritten extents

Filesystem behavior is pretty open-ended for sparse ranges (i.e. holes)
of a file that have not yet been written to.  That space can remain
unmapped, it can be mapped to written space that has been zeroed, or it
can be mapped to unwritten extents.

This program trips over that last condition on XFS.  If the file is
located on a data device with a raid stripe geometry or on a realtime
device with a realtime extent size larger than 1 filesystem block, it's
possible for unwritten areas to be backed by unwritten preallocations or
unwritten rt blocks, respectively.

Fix the test to skip unwritten extents here.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>