zfs_netbsd_{create,mknod,link,etc..} that call functions called
zfs_{create,mknod,link,etc..}. These later functions may return a
error code along with a *vpp that is NULL. This situation was not
handled by the zfs_netbsd_* functions and would result in a panic in a
number of cases. The simplest to trigger it was filling up a dataset
or pool resulting in a over quota condition. An attempt to create
another file, or directory at that point would panic.