fs: relax memory alignment restriction for O_DIRECT

fs: relax memory alignment restriction for O_DIRECT

We currently require blocksize alignment for both position/length and the
memory range. The latter is overly restrictive, as most devices can do DMA
at a much finer granularity.

Honor the bdev setting for DMA alignment, and use that for checking if we
can process the IO or need to error it.

Signed-off-by: Jens Axboe <axboe@kernel.dk>