generic: Another SEEK_HOLE/SEEK_DATA sanity test

generic: Another SEEK_HOLE/SEEK_DATA sanity test

Both ext4 and xfs have a bug in the page cache scanning code for
SEEK_HOLE / SEEK_DATA in unwritten extents: the start offset isn't
taken into account when scanning a page, so seeking can fail on
filesystems with a block size less than half of the page size.  For
example, the following command fails on a filesystem with a block
size of 1k:

  xfs_io -f -c "falloc 0 4k" \
            -c "pwrite 1k 1k" \
            -c "pwrite 3k 1k" \
            -c "seek -a -r 0" foo

Like with generic/436, the actual tests are added to seek_sanity_test.

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