ksmbd: remove filename in ksmbd_file

If the filename is change by underlying rename the server, fp->filename
and real filename can be different. This patch remove the uses of
fp->filename in ksmbd and replace it with d_path().

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>