ksmbd: make __dir_empty() compatible with POSIX

Some file systems may not provide dot (.) and dot-dot (..) as they are
optional in POSIX. ksmbd can misjudge emptiness of a directory in those
file systems, since it assumes there are always at least two entries:
dot and dot-dot.
Just don't count dot and dot-dot.

Cc: stable@vger.kernel.org # v6.1+
Signed-off-by: Hobin Woo <hobin.woo@samsung.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>