pgsql: refactor: Move BackendWritebackContext initialization

refactor: Move BackendWritebackContext initialization

BackendWritebackContext is a purely per-process thing, so it feels
wrong to initialize it in the shmem init/attach functions. Move the
initialization to InitBufferManagerAccess() instead.

Backpatch to version 19, where the new shared memory callbacks system
was introduced. While this isn't new with that, it made the ShmemInit
functions more closely related to shared memory initialization than
before.

Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/fb1bd612-4a58-4235-976f-2b2ef036bfff@iki.fi
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1feb619383d05395936c6c52144500c7becb409e

Modified Files
--------------
src/backend/storage/buffer/buf_init.c | 15 ---------------
src/backend/storage/buffer/bufmgr.c | 6 ++++++
src/include/storage/buf_internals.h | 1 -
3 files changed, 6 insertions(+), 16 deletions(-)