RefreshVdevState() iterates through all the system's zpools,
		which involves the ioctls ZFS_IOC_POOL_CONFIGS and
		ZFS_IOC_POOL_STATS.  Both of those acquire
		spa_namespace_lock, which may block for a long time under
		certain circumstances, including when the system has a storm
		of IO or checksum errors.  This change eliminates the call
		to RefreshVdevState() whenever a ZFSEvent is received.
		Instead, RefreshVdevState() will only be called when a
		CaseFile is closed, if necessary.  This way, zfsd won't
		spend too much time blocking on ioctl()s and miss reading
		events from devd.