Fix formula in _hash_spareindex.

This was correct in earlier versions of the patch that lead to
commit ea69a0de, but somehow got
broken in the last version which I actually committed.

Mithun Cy, per an off-list report from Ashutosh Sharma

Discussion: http://postgr.es/m/CAD__OujbAwNU71v1y-RoQxZ8LZ6-V2UFTkex3v34MK6uZ3Xb5w@mail.gmail.com

src/backend/access/hash/hashutil.c

@@ -174,7 +174,8 @@ _hash_spareindex(uint32 num_bucket)
 
 	/* account for phases within current group */
 	splitpoint_phases +=
-		(((num_bucket - 1) >> (HASH_SPLITPOINT_PHASE_BITS + 1)) &
+		(((num_bucket - 1) >>
+		  (splitpoint_group - (HASH_SPLITPOINT_PHASE_BITS + 1))) &
 		 HASH_SPLITPOINT_PHASE_MASK);	/* to 0-based value. */
 
 	return splitpoint_phases;