[MASSMAIL]pgsql: Fix zeroing of pg_serial page without SLRU bank lock

Fix zeroing of pg_serial page without SLRU bank lock

Bug in commit 53c2a97a9266: we failed to acquire the correct SLRU bank
lock when iterating to zero-out intermediate pages in predicate.c.
Rewrite the code block so that we follow the locking protocol correctly.

Also update an outdated comment in the same file -- SerialSLRULock
exists no more.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Discussion: https://postgr.es/m/2a25eaf4-a3a4-5fd1-6241-9d7c73142085@gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/be2f0731006300876b008fd78a265471984cbcd9

Modified Files
--------------
src/backend/storage/lmgr/predicate.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)