zvol_os: Properly ignore error in volmode lookup

We fall back to a default volmode and continue when looking up a zvol's
volmode property fails. After this we should set the error to 0 to
ensure we take the success paths in the out section.

While here, make sure we only log that the zvol was created on success.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11117