Add a sysctl to allow ZFS pools backed by zvols

Change 294329 removed the ability to build ZFS pools that are backed by
zvols, because having that ability (even if it's not used) leads to
deadlocks. By popular demand, I'm adding an off-by-default sysctl to
reenable that ability.

Reviewed by: lidl, delphij
MFC after: Never
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D4998