aiodio_sparse2: fix up alignment for 4k logical block sized devices

aiodio_sparse2: fix up alignment for 4k logical block sized devices

When running xfstests on a 4k logical sector device, I ran into a test
failure in test 198.  The errors were all due to trying to do 512 byte
aligned I/O on a 4k logical sector device.  The attached patch tries to
auto-detect the proper block size if no alignment is specified.  If it
fails for one reason or another, it defaults to 4k alignment.  This
seems to work fine in my test rig.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>