ksmbd: factor out a ksmbd_vfs_lock_parent helper

Factor out a self-contained helper to
get stable parent dentry.

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>