ksmbd: fix open failure from block and char device file

char/block device file can't be opened with dentry_open() if device driver
is not loaded. Use O_PATH flags for fake opening file to handle it if file
is a block or char file.

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