src/seek_sanity_test: fix test15 SEEK_HOLE expected results

src/seek_sanity_test: fix test15 SEEK_HOLE expected results

Filesystesm with the "default behavior" will always return the
offset of the end of the file when lseek'ing with SEEK_HOLE.  This
test does the following:

 - fallocate 4 << 20 bytes
 - write PAGE_SIZE bytes at offset 0
 - writes PAGE_SIZE bytes at offset 4 << 20

Thus, using lseek in an FS with the "default behavior" will set the
position at 4 << 20 + PAGE_SIZE.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>