Speed up "zpool import" in the presence of many zvols

By default, FreeBSD does not allow zpools to 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, or a zvol with
many snapshots.

https://svnweb.freebsd.org/base?view=revision&revision=357235
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241083
https://reviews.freebsd.org/D22077

Obtained from: FreeBSD
Reported by: Martin Birgmeier <d8zNeCFG@aon.at>
Sponsored by: Axcient
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #11502