pgsql: Fix breakage by the wal_sync_method patch in installations that

Log Message:
-----------
Fix breakage by the wal_sync_method patch in installations that use
O_DSYNC (specifically this broke all the Windows buildfarm members)

Modified Files:
--------------
pgsql/src/backend/access/transam:
xlog.c (r1.306 -> r1.307)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.306&r2=1.307)