cifsd: fix wrong prototype in comment

kernel test robot reported:
 >> fs/cifsd/oplock.c:1454: warning: expecting prototype for
    create_durable_rsp__buf(). Prototype was for
    create_durable_rsp_buf() instead

This patch fix wrong prototype in comment.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>