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@gmail.com>
Discussion: https://postgr.es/m/CALdSSPgBPnpNNzxv0Y+_GNFzW6PmzRZYh+_hpf06Y1N2zLhZaQ@mail.gmail.com