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

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/commands:
        vacuumlazy.c (r1.115 -> r1.116)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.115&r2=1.116)