src/seek_sanity_test: Fix for filesystems without unwritten extent support

src/seek_sanity_test: Fix for filesystems without unwritten extent support

src/seek_sanity_test assumes that after preallocating space in a
file with fallocate, fseek SEEK_HOLE / SEEK_DATA will still report
the allocated space as a hole.  On filesystems without unwritten
extent support, that space will be reported as data, though.  On
such filesystems, skip the unwritten extent tests.

Tested on ext4, xfs, and gfs2 + patches for fseek SEEK_HOLE /
SEEK_DATA support.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>