pgsql: Fix logic in lazy vacuum to decide if it's worth trying to

Log Message:
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Fix logic in lazy vacuum to decide if it's worth trying to truncate the heap.
If the table was smaller than REL_TRUNCATE_FRACTION (= 16) pages, we always
tried to acquire AccessExclusiveLock on it even if there was no empty pages
at the end.

Report by Simon Riggs. Back-patch all the way to 7.4.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/commands:
        vacuumlazy.c (r1.61.2.6 -> r1.61.2.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.61.2.6&r2=1.61.2.7)