pgsql: Sanitize some WAL-logging buffer handling in GIN and GiST code

Sanitize some WAL-logging buffer handling in GIN and GiST code

As transam's README documents, the general order of actions recommended
when WAL-logging a buffer is to unlock and unpin buffers after leaving a
critical section. This pattern was not being followed by some code
paths of GIN and GiST, adjusted in this commit, where buffers were
either unlocked or unpinned inside a critical section. Based on my
analysis of each code path updated here, there is no reason to not
follow the recommended unlocking/unpin pattern done outside of a
critical section.

These inconsistencies are rather old, coming mainly from ecaa4708e5dd
and ff301d6e690b. The guidelines in the README predate these commits,
being introduced in 6d61cdec0761.

Author: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Discussion: https://postgr.es/m/CALdSSPgBPnpNNzxv0Y+_GNFzW6PmzRZYh+_hpf06Y1N2zLhZaQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/21e323e941cf6ff02f931cd21bcfe4ab2cb5d622

Modified Files
--------------
src/backend/access/gin/gindatapage.c | 4 ++--
src/backend/access/gin/ginfast.c | 12 ++++++------
src/backend/access/gin/ginutil.c | 4 ++--
src/backend/access/gin/ginvacuum.c | 6 +++---
src/backend/access/transam/xloginsert.c | 7 ++++---
5 files changed, 17 insertions(+), 16 deletions(-)