xfs: split the unaligned DIO write code out

The unaligned DIO write path is more convolted than the normal path,
and we are about to make it more complex. Keep the block aligned
fast path dio write code trim and simple by splitting out the
unaligned DIO code from it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
[hch: rebased, fixed a few minor nits]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>