pgsql: Fix past pd_upper write in ginRedoRecompress()

Fix past pd_upper write in ginRedoRecompress()

ginRedoRecompress() replays actions over compressed segments of posting list
in-place.  However, it might lead to write past pg_upper, because intermediate
state during playing the changes can take more space than both original state
and final state.  This commit fixes that by refuse from in-place modification.
Instead page tail is copied once modification is started, and then it's used
as the source of original segments.  Backpatch to 9.4 where posting list
compression was introduced.

Reported-by: Sivasubramanian Ramasubramanian
Discussion: https://postgr.es/m/1536091151804.6588%40amazon.com
Author: Alexander Korotkov based on patch from and ideas by Sivasubramanian Ramasubramanian
Review: Sivasubramanian Ramasubramanian
Backpatch-through: 9.4

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f9e66f2fbbb49a493045c8d8086a9b15d95b8f18

Modified Files
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src/backend/access/gin/ginxlog.c | 80 +++++++++++++++++++++++++++++-----------
1 file changed, 59 insertions(+), 21 deletions(-)