zvol_os: Keep better track of open count in close

zvol_geom_close gets a count of the number of close operations to do.

Make sure we're always using this count to check if this will be the
last close operation performed on the zvol.

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