cifs: Fix potential OOB access of lock element array

If maxBuf is small but non-zero, it could result in a zero sized lock
element array which we would then try and access OOB.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>