Only has two callers, and is just a shutdown check and error handler
around xfs_buf_iorequest. However, the error handling is a mess of
read and write semantics, and both internal callers only call it for
writes. Hence kill the wrapper, and follow up with a patch to
sanitise the error handling.
Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Dave Chinner <david@fromorbit.com>