That caused a deadlock because the manipulations of zvol devices
is done under spa_namespace_lock, but there are cases where the
lock is held while waiting for the sync thread.
This is imperfect, because zvol updates are done with an arbitrary
delay after the corresponding dataset changes. So, potentially
zvol devices can get out of sync if multiple changes are performed
A better solution would be to queue all changes and apply them to zvol
devices in the same order as they are originally done.