Speed up "zpool import" in the presence of many zvols
By default, zpools may not be backed by zvols (that can be changed with the
"vfs.zfs.vol.recursive sysctl). When that sysctl is set to 0, the kernel
does not attempt to read zvols when looking for vdevs. But the zpool
command still does. This change brings the zpool command into line with the
kernel's behavior. It speeds "zpool import" when an already imported pool
has many zvols.
Reported by: Martin Birgmeier <d8zNeCFG@aon.at>