seek_sanity_test: Repair check for unwritten extent support

seek_sanity_test: Repair check for unwritten extent support

In test_basic_support, commit f3c1bca7fb25 ("generic: Test that
SEEK_HOLE can find a punched hole") cleverly punched a hole in the test
file in the middle of the check for unwritten extent support, making
sure we would never detect when unwritten extent support is missing.
Fix that.

While at it, explicitly check for SEEK_DATA support as well:  so far, we
were assuming that SEEK_HOLE support implies SEEK_DATA support, but it
won't hurt to actually check.

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