ksmbd: replace generic_fillattr with vfs_getattr

generic_fillattr should not be used outside of ->getattr
implementations.

Use vfs_getattr instead, and adapt functions to return an
error code to the caller.

Cc: stable@vger.kernel.org
Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>