BACKPORT: FROMGIT: 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.

(cherry picked from commit 154a03dac54fb49e4068b58a1005eb2bd02a5dc4
 https://github.com/smfrench/smb3-kernel.git

 ksmbd-for-next-next)
[hobin: Replace non-present helper in fs/smb/server/vfs.c ]
Change-Id: I0fd226dd3f287f6ff7ef12c5e9f0a986f75b59b1
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Hobin Woo <hobin.woo@samsung.com>