FreeBSD: Fix spurious failure in zvol_geom_open

In zvol_geom_open on first open we need to guarantee
that the namespace lock is held to avoid spurious
failures in zvol_first_open.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10841