pgsql: bufmgr: Fix order of operations in UnlockBufHdrExt

bufmgr: Fix order of operations in UnlockBufHdrExt

In c75ebc657ffc I (Andres) introduced UnlockBufHdrExt() which can set and
clear bits in the buffer state using CAS. Unfortunately I added bits before
subtracting them, which means that a bit that was both removed and set would
remain unset. Fix the order of operations.

The only known case where that is a problem is that BM_IO_ERROR would not
actually remain set.

It's unfortunately not trivial to add a decent, race-free, test to verify that
BM_IO_ERROR remains set. That's therefore left for the 20 cycle.

Reported-by: Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru>
Discussion: https://postgr.es/m/ab0dcc9e-aba0-44e3-ac23-8d74c48888e6@postgrespro.ru
Backpatch-through: 19, where c75ebc657ffc went in

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c787c952bc882626c66ec64b1d35a664902bfa8b

Modified Files
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src/include/storage/buf_internals.h | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)