cifs: warn about deprecation of /proc/fs/cifs/OplockEnabled interface

The plan is to deprecate this interface by kernel version 3.4.

Changes since v1
   - add a '\n' to the printk.

Reported-by: Alexander Swen <alex@swen.nu>
Cc: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <smfrench@gmail.com>