pgsql: Prevent deadlock in ginRedoDeletePage()

Prevent deadlock in ginRedoDeletePage()

On standby ginRedoDeletePage() can work concurrently with read-only queries.
Those queries can traverse posting tree in two ways.
1) Using rightlinks by ginStepRight(), which locks the next page before
   unlocking its left sibling.
2) Using downlinks by ginFindLeafPage(), which locks at most one page at time.

Original lock order was: page, parent, left sibling.  That lock order can
deadlock with ginStepRight().  In order to prevent deadlock this commit changes
lock order to: left sibling, page, parent.  Note, that position of parent in
locking order seems insignificant, because we only lock one page at time while
traversing downlinks.

Reported-by: Chen Huajun
Diagnosed-by: Chen Huajun, Peter Geoghegan, Andrey Borodin
Discussion: https://postgr.es/m/31a702a.14dd.166c1366ac1.Coremail.chjischj%40163.com
Author: Alexander Korotkov
Backpatch-through: 9.4

Branch
------
REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/80d4d8d71d57cc336e00e448f14c1b70ad8bc85f

Modified Files
--------------
src/backend/access/gin/ginxlog.c | 22 +++++++++++++---------
1 file changed, 13 insertions(+), 9 deletions(-)