Drop of spa_namespace lock in vdev_geom.c

[Justin T. Gibbs]

Dropping and reacquiring the spa_namespace lock in vdev_geom_open()
creates a lock order reversal with the spa_config locks.  As the
spa_config locks are not standard mutexes, witness will not warn
about this issue.  I only noticed this problem when debugging a ZFS
deadlock.  The deadlock can be triggered anytime that there are
multiple insert/remove processes going on (e.g. vdev orphan processing
while a fault management daemon is onlining a replacement device for
some other vdev).

I haven't noticed any issues with just holding the namespace lock
for the duration of the open.  Does anyone know why this lock drop
was added in v28?

Thanks,
Justin