dio: create a dio_aligned() helper function

dio: create a dio_aligned() helper function

__blockdev_direct_IO() had two instances of the same code to determine
if a given offset wasn't aligned first to the inode's blkbits and then
to the underlying device's blkbits.   This was confusing enough but
we're about to add code that performs the same check on offsets in bvec
arrays.  Rather than add yet more copies of this code let's have
everyone call a helper.

Signed-off-by: Zach Brown <zach.brown@oracle.com>