pgsql: Fix traversing to the deleted GIN page via downlink

Fix traversing to the deleted GIN page via downlink

Current GIN code appears to don't handle traversing to the deleted page via
downlink.  This commit fixes that by stepping right from the delete page like
we do in nbtree.

This commit also fixes setting 'deleted' flag to the GIN pages.  Now other page
flags are not erased once page is deleted.  That helps to keep our assertions
true if we arrive deleted page via downlink.

Discussion: https://postgr.es/m/CAPpHfdvMvsw-NcE5bRS7R1BbvA4BxoDnVVjkXC5W0Czvy9LVrg%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Peter Geoghegan
Backpatch-through: 9.4

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ab64b474d9bec8486b42e550526a121f0d81b681

Modified Files
--------------
src/backend/access/gin/ginbtree.c    | 7 -------
src/backend/access/gin/gindatapage.c | 3 +++
src/backend/access/gin/ginvacuum.c   | 2 +-
src/backend/access/gin/ginxlog.c     | 2 +-
4 files changed, 5 insertions(+), 9 deletions(-)