pgsql: Remove BTScanOpaqueData.firstPage

Remove BTScanOpaqueData.firstPage

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/06b10f80ba4db745f21847520b07b4ffad814313

Modified Files
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src/backend/access/nbtree/nbtree.c    |  1 -
src/backend/access/nbtree/nbtsearch.c | 18 ++++++++----------
src/include/access/nbtree.h           |  3 ---
3 files changed, 8 insertions(+), 14 deletions(-)