xfs: fix error handling at xfs_bulkstat

From: Jie Liu <jeff.liu@oracle.com>

We should not ignore the btree operation errors at xfs_bulkstat() but
to propagate them if any. This patch fix two places in this function
and the remaining things will be fixed with code refactoring thereafter.

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