pgsql: Fix handling of all-zero pages in SP-GiST vacuum.

Fix handling of all-zero pages in SP-GiST vacuum.

SP-GiST initialized an all-zeros page at vacuum, but that was not
WAL-logged, which is not safe. You might get a torn page write, when it gets
flushed to disk, and end-up with a half-initialized index page. To fix,
leave it in the all-zeros state, and add it to the FSM. It will be
initialized when reused. Also don't set the page-deleted flag when recycling
an empty page. That was also not WAL-logged, and a torn write of that would
cause the page to have an invalid checksum.

Backpatch to 9.2, where SP-GiST indexes were added.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/023430abf72eb7d335430e241065d5ed19ddd94b

Modified Files
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src/backend/access/spgist/spgvacuum.c | 27 ++++++++-------------------
src/include/access/spgist_private.h | 4 ++--
2 files changed, 10 insertions(+), 21 deletions(-)