Fix incorrect math in DetermineSafeOldestOffset.

The old formula didn't have enough parentheses, so it would do the wrong
thing, and it used / rather than % to find a remainder.  The effect of
these oversights is that the stop point chosen by the logic introduced in
commit b69bf30b might be rather
meaningless.

Thomas Munro, reviewed by Kevin Grittner, with a whitespace tweak by me.

src/backend/access/transam/multixact.c

@@ -2495,7 +2495,8 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
 	 */
 	oldestOffset = find_multixact_start(oldestMXact);
 	/* move back to start of the corresponding segment */
-	oldestOffset -= oldestOffset / MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT;
+	oldestOffset -= oldestOffset %
+		(MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
 
 	LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 	/* always leave one segment before the wraparound point */