[SPARK-5282][mllib]: RowMatrix easily gets int overflow in the memory size warning

JIRA: https://issues.apache.org/jira/browse/SPARK-5282

fix the possible int overflow in the memory computation warning

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #4069 from hhbyyh/addscStop and squashes the following commits:

e54e5c8 [Yuhao Yang] change to MB based number
7afac23 [Yuhao Yang] 5282: fix int overflow in the warning

mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/RowMatrix.scala

@@ -131,8 +131,8 @@ class RowMatrix(
       throw new IllegalArgumentException(s"Argument with more than 65535 cols: $cols")
     }
     if (cols > 10000) {
-      val mem = cols * cols * 8
-      logWarning(s"$cols columns will require at least $mem bytes of memory!")
+      val memMB = (cols.toLong * cols) / 125000
+      logWarning(s"$cols columns will require at least $memMB megabytes of memory!")
     }
   }