generic/223, xfs/203: IO is not well aligned

generic/223, xfs/203: IO is not well aligned

On certain configurations (e.g. MOUNT_OPTIONS="-o dax") we get
different allocation patterns due to the writes being done in
multiple pwrite() calls. e.g. the write is 8k, but the buffer size
is 4k, and so the filesystem sees 4k writes. If the filesytem is not
using delayed allocation, then the allocation context is a 4k write
rather than an 8k write and so they don't get appropriately aligned.

Fix this by making the write buffer the same size and the writes
being done.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>