ZFS: move zvol device operation out of txg sync thread

[Anthoine Bourgeois]

Hello,

I posted the review https://reviews.freebsd.org/D14669

I wonder if I should insert the command in the queue earlier in the
functions or should it be fine to be close to the dsl_dataset operations as
it is currently ?

Any review is welcome.

Another question: I create a new review D14669 because I can't find how
to update the review D7179, that may be more proper I think. When I
click on "Update diff" and select my diff file, the "Attach To" list only
show me the "Create a new revision..." option. Is is normal ?

Best,
Anthoine