CIFS: Fix wrong filename length for SMB2

The existing code uses the old MAX_NAME constant. This causes
XFS test generic/013 to fail. Fix it by replacing MAX_NAME with
PATH_MAX that SMB1 uses. Also remove an unused MAX_NAME constant
definition.

Cc: <stable@vger.kernel.org> # v3.7+
Signed-off-by: Pavel Shilovsky <pshilovsky@samba.org>
Signed-off-by: Steve French <smfrench@gmail.com>