Improving zvol support

[Steven Hartland]

There's a few outstanding PR's for zvol issues such as:-
http://www.freebsd.org/cgi/query-pr.cgi?pr=178999

I've attached a patch to this PR (also to this email)
which helps clear quite a few issues highlighted by this
PR and by tests in STF.

The locking side of this patch while not 100% ideal does
prevent the common issues such as zvol renames so I think
worth getting this committing until more time can be spent
on it.

What do people think?

    Regards
    Steve
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