xfs: add discontiguous buffer support to transactions

Now that the buffer cache supports discontiguous buffers, add
support to the transaction buffer interface for getting and reading
buffers.

Note that this patch does not convert the buffer item logging to
support discontiguous buffers. That will be done as a separate
commit.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>