xfs: lobotomise xfs_trans_read_buf_map()

There's a case in that code where it checks for a buffer match in a
transaction where the buffer is not marked done. i.e. trying to
catch a buffer we have locked in the transaction but have not
completed IO on.

The only way we can find a buffer that has not had IO completed on
it is if it had readahead issued on it, but we never do readahead on
buffers that we have already joined into a transaction. Hence this
condition cannot occur, and buffers locked and joined into a
transaction should always be marked done and not under IO.

Remove this code and re-order xfs_trans_read_buf_map() to remove
duplicated IO dispatch and error handling code.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>