Fix RBM_ZERO_AND_LOCK.

Fix RBM_ZERO_AND_LOCK.

Commit 210622c6 accidentally zeroed out pages even if they were found in
the buffer pool.  It should always lock the page, but it should only
zero pages that were not already valid.  Otherwise, concurrent readers
that hold only a pin could see corrupted page contents changing under
their feet.

While here, rename ZeroAndLockBuffer() to match the RBM_ flag name.
Also restore a some useful comments lost by 210622c6's refactoring, and
add some new ones to clarify why we need to use the BM_IO_IN_PROGRESS
infrastructure despite not doing I/O.

Reported-by: Noah Misch 
Reported-by: Alexander Lakhin 
Reviewed-by: Alvaro Herrera  (earlier version)
Reviewed-by: Robert Haas  (earlier version)
Discussion: https://postgr.es/m/20240512171658[email protected]
Discussion: https://postgr.es/m/7ed10231-ce47-03d5-d3f9-4aea0dc7d5a4%40gmail.com