It's not necessary to keep the firstPage flag as a field of BTScanOpaqueData.
This commit makes it an argument of the _bt_readpage() function. We can easily
distinguish first-time and repeated calls (within the scan) of this function.
Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzk4SOsw%2BtHuTFiz8U9Jqj-R77rYPkhWKODCBb1mdHACXA%40mail.gmail.com
Reviewed-by: Pavel Borisov