MFC r318925: MFV r316929: 6914 kernel virtual memory fragmentation leads to hang

illumos/illumos-gate@af868f46a5b794687741d5424de9e3a2d684a84a
https://github.com/illumos/illumos-gate/commit/af868f46a5b794687741d5424de9e3a2d684a84a

https://www.illumos.org/issues/6914

FreeBSD note: only a ZFS part of the change is merged, changes to the VM
subsystem are not ported (obviously). Also, now that FreeBSD has
vmem(9) we don't have to ifdef-out the code that uses it.