zvol locking fixes

[Steven Hartland]

Hi guys, zvol locking fixes have been mentioned a few times
before now, during which the guys at SpectraLogic mentioned
they had some in depth changes which fix the problems and
where working on getting them into the tree.

I've been sitting on a number of PR's for a long time now,
for which I have some fixes namely kern/181235 & kern/178999
awaiting these fixes but to my knowledge they have yet to be
commited.

Could we please get an update on this as if its not going
to make it into the tree in the near future I'm going to
commit my fixes which at least help with some of the common
cases.

    Regards
    Steve