src/min_dio_alignment: don't return invalid dio_offset_align

src/min_dio_alignment: don't return invalid dio_offset_align

If returned parameters of statx() are: a)STATX_DIOALIGN is set in
stx_mask, b)stx.stx_dio_offset_align is zero, it indicates filesystem
supports DIO, but the file doesn't.

It needs to avoid returning zeroed stx.stx_dio_offset_align value,
instead, we can fallthrough to get alignment size of block device or
page size, otherwise, it may cause potential deadloop, e.g.
generic/465:

align=stx_dio_offset_align(it equals to zero)
page_size=4096
while [ $align -le $page_size ]; do
    echo "$AIO_TEST -a $align -d $testfile.$align" >> $seqres.full
    $AIO_TEST -a $align -d $testfile.$align 2>&1 | tee -a $seqres.full
    align=$((align * 2))
done

Cc: Christoph Hellwig <hch@lst.de>
Cc: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>