pgsql: Minimize footprint of TOAST_MAX_CHUNK_SIZE in heap and amcheck c

Minimize footprint of TOAST_MAX_CHUNK_SIZE in heap and amcheck code

This refactoring eases a little bit a follow-up change to add support
for 8-byte TOAST value IDs, as the maximum chunk size allowed for a
single chunk of TOASTed data depends on the size of varatt_external, its
value ID being part of it.

It is worth noting that TOAST_MAX_CHUNK_SIZE resolves as a uintptr_t, so
the previous calculations were in 64-bit unsigned arithmetic. These are
now done in int32, which still works the same way due to an external
TOAST value being bounded to 1GB.

Author: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/CALj2ACVnO6jjs5aRx3dSOBy9LV7=iakPBR1eOvoeB8rL8Q47zg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2b3d11aaed891958f24aed05bf454524d603b6ea

Modified Files
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contrib/amcheck/verify_heapam.c | 13 +++++++++----
src/backend/access/heap/heaptoast.c | 20 ++++++++++++--------
2 files changed, 21 insertions(+), 12 deletions(-)