ksmbd: move leading slash check to smb2_get_name()

If the directory name in the root of the share starts with
character like 镜(0x955c) or Ṝ(0x1e5c), it (and anything inside)
cannot be accessed. The leading slash check must be checked after
converting unicode to nls string.

Cc: stable@vger.kernel.org
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>