Bandaid over export of zvol when zed is not running.
vdev_disk's vnode_open would use vfs_context_current() and effectively
attempt to open disk as user. This would make "zpool scrub" hang due
to failure to open in zio pipeline.
Changed to use vfs_context_kernel() to match upstreams use of "kcred".
Also change vdev_disk's close and flushcache calls use vfs_context_kernel().