pgsql: Get heap page max offset with buffer lock held.

Get heap page max offset with buffer lock held.

On further reflection it seems better to call PageGetMaxOffsetNumber()
after acquiring a buffer lock on the page.  This shouldn't really
matter, but doing it this way is cleaner.

Follow-up to commit 42288174.

Backpatch: 12-, just like commit 42288174

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/32d6287d2eef6b6a4dde07e0513f3e4f321792ad

Modified Files
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src/backend/access/heap/heapam.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)