In dsm_impl_windows, don't error out when the segment already exists.

This is the behavior of the other implementations, and the behavior
expected by the callers of this function.

Amit Kapila

src/backend/storage/ipc/dsm_impl.c

@@ -694,10 +694,6 @@ dsm_impl_windows(dsm_op op, dsm_handle handle, uint64 request_size,
 			 * modified.
 			 */
 			CloseHandle(hmap);
-			ereport(elevel,
-					(errcode_for_dynamic_shared_memory(),
-					 errmsg("could not open shared memory segment \"%s\": %m",
-						name)));
 			return false;
 		}
 	}