Diffusion FreeBSD src repository - subversion rS362047

rework how ZVOLs are updated in response to DSL operations
rS362047
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Description

rework how ZVOLs are updated in response to DSL operations

With this change all ZVOL updates are initiated from the SPA sync
context instead of a mix of the sync and open contexts. The updates are
queued to be applied by a dedicated thread in the original order. This
should ensure that ZVOLs always accurately reflect the corresponding
datasets. ZFS ioctl operations wait on the mentioned thread to complete
its work. Thus, the illusion of the synchronous ZVOL update is
preserved. At the same time, the SPA sync thread never blocks on ZVOL
related operations avoiding problems like reported in bug 203864.

This change is based on earlier work in the same direction: D7179 and
D14669 by Anthoine Bourgeois. D7179 tried to perform ZVOL operations
in the open context and that opened races between them. D14669 uses a
design very similar to this change but with different implementation
details.

This change also heavily borrows from similar code in ZoL, but there are
many differences too. See:

PR: 203864
MFC after: 5 weeks
Sponsored by: CyberSecure
Differential Revision: https://reviews.freebsd.org/D23478