pgsql: Support RBM_ZERO_AND_CLEANUP_LOCK in ExtendBufferedRelTo(), add

Support RBM_ZERO_AND_CLEANUP_LOCK in ExtendBufferedRelTo(), add tests

For some reason I had not implemented RBM_ZERO_AND_CLEANUP_LOCK support in
ExtendBufferedRelTo(), likely thinking it not being reachable. But it is
reachable, e.g. when replaying a WAL record for a page in a relation that
subsequently is truncated (likely only reachable when doing crash recovery or
PITR, not during ongoing streaming replication).

As now all of the RBM_* modes are supported, remove assertions checking mode.

As we had no test coverage for this scenario, add a new TAP test. There's
plenty more that ought to be tested in this area...

Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Reviewed-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Discussion: https://postgr.es/m/392271.1681238924%40sss.pgh.pa.us
Discussion: https://postgr.es/m/0b5eb82b-cb99-e0a4-b932-3dc60e2e3926@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/43a33ef54e503b61f269d088f2623ba3b9484ad7

Modified Files
--------------
src/backend/storage/buffer/bufmgr.c | 28 +++---
src/test/recovery/meson.build | 1 +
src/test/recovery/t/036_truncated_dropped.pl | 136 +++++++++++++++++++++++++++
3 files changed, 154 insertions(+), 11 deletions(-)