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, or a zvol with many snapshots.

PR: 241083
Reported by: Martin Birgmeier <d8zNeCFG@aon.at>
Reviewed by: mav, Ryan Moeller <ryan@freqlabs.com>
MFC after: 2 weeks
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D22077