ksmbd: constify struct path

... in particular, there should never be a non-const pointers to
any file->f_path.

Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steve French <stfrench@microsoft.com>