pgsql: Fix is-it-time-for-a-checkpoint logic so that checkpoint_segments

Log Message:
-----------
Fix is-it-time-for-a-checkpoint logic so that checkpoint_segments can
usefully be larger than 255. Per gripe from Simon Riggs.

Modified Files:
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pgsql/src/backend/access/transam:
xlog.c (r1.178 -> r1.179)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.178&r2=1.179)