fs: cifs: Fix atime update check vs mtime

According to the comment in the code and commit log, some apps
expect atime >= mtime; but the introduced code results in
atime==mtime.  Fix the comparison to guard against atime<mtime.

Fixes: 9b9c5bea ("cifs: do not return atime less than mtime")
Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: stfrench@microsoft.com
Cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>