cifsd: re-implement ksmbd_vfs_kern_path

re-implement ksmbd_vfs_kern_path() to change
recursion to iteration.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>