pgsql: Remove dubious micro-optimization inckpt_buforder_comparator().

Remove dubious micro-optimization in ckpt_buforder_comparator().

It seems incorrect to assume that the list of CkptSortItems can never
contain duplicate page numbers: concurrent activity could result in some
page getting dropped from a low-numbered buffer and later loaded into a
high-numbered buffer while BufferSync is scanning the buffer pool.
If that happened, the comparator would give self-inconsistent results,
potentially confusing qsort().  Saving one comparison step is not worth
possibly getting the sort wrong.

So far as I can tell, nothing would actually go wrong given our current
implementation of qsort().  It might get a bit slower than expected
if there were a large number of duplicates of one value, but that's
surely a probability-epsilon case.  Still, the comment is wrong,
and if we ever switched to another sort implementation it might be
less forgiving.

In passing, avoid casting away const-ness of the argument pointers;
I've not seen any compiler complaints from that, but it seems likely
that some compilers would not like it.

Back-patch to 9.6 where this code came in, just in case I've underestimated
the possible consequences.

Discussion: https://postgr.es/m/18437.1515607610@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c24a908d04806c34c3e5dda42d39bb5ca1fc5ffb

Modified Files
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src/backend/storage/buffer/bufmgr.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)