MFV r323795: 8604 Avoid unnecessary work search in VFS when unmounting snapshots

illumos/illumos-gate@ed992b0aac4e5b70dc1273b1d055c0d471fbb4b1
https://github.com/illumos/illumos-gate/commit/ed992b0aac4e5b70dc1273b1d055c0d471fbb4b1

https://www.illumos.org/issues/8604

Every time we want to unmount a snapshot (happens during snapshot deletion or
renaming) we unnecessarily iterate through all the mountpoints in the VFS layer
(see zfs_get_vfs).
Ideally we would just put a hold on the snapshot and access its respective VFS
resource directly.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>

FreeBSD note: I added a FreeBSD specific function getzfsvfs_ref() which
is like getzfsvfs() but returns a filesystem referenced, not busied.
We want a busied filesystem in most cases, because we access its private
data and, thus, we need to prevent the filesystem from being unmounted
and its private data destroyed. But in some cases we can either get
away with just a referenced filesystem or we must not busy the
filesystem. Unmounting the filesystem is one of such cases.

MFC after: 5 weeks
X-MFC after: r324163