xfs: kill xfs_bioerror_relse

There is only one caller now - xfs_trans_read_buf_map() - and it has
very well defined call semantics - read, synchronous, and b_iodone
is NULL. Hence it's pretty clear what error handling is necessary
for this case. The bigger problem of untangling
xfs_trans_read_buf_map error handling is left to a future patch.

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