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
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c0bf35421529ab210479ea0e828270e6626adff6

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(-)