Add missing dmu_zfetch_fini() in dnode_move_impl()

As it turns out, on the Windows platform when rw_init() is called
(rather its bedrock call ExInitializeResourceLite) it is placed on
an active-list of locks, and is removed at rw_destroy() time.

dnode_move() has logic to copy over the old-dnode to new-dnode,
including calling dmu_zfetch_init(new-dnode). But due to the missing
dmu_zfetch_fini(old-dnode), kmem will call dnode_dest() to release the
memory (and in debug builds fill pattern 0xdeadbeef) over the Windows
active-lock's prev/next list pointers, making Windows sad.

But on other platforms, the contents of dmu_zfetch_fini() is one
call to list_destroy() and one to rw_destroy(), which is effectively
a no-op call and is not required. This commit is mostly for
"correctness" and can be skipped there.

Porting Notes:
* This leak exists on Linux but currently can never happen because
  the dnode_move() functionality is not supported.

openzfsonosx-commit: openzfsonosx/zfs@d95fe517

Authored by: Julian Heuking <JulianH@beckhoff.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #8519

module/zfs/dnode.c

@@ -840,7 +840,7 @@ dnode_move_impl(dnode_t *odn, dnode_t *ndn)
 	    offsetof(dmu_buf_impl_t, db_link));
 	odn->dn_dbufs_count = 0;
 	odn->dn_bonus = NULL;
-	odn->dn_zfetch.zf_dnode = NULL;
+	dmu_zfetch_fini(&odn->dn_zfetch);
 
 	/*
 	 * Set the low bit of the objset pointer to ensure that dnode_move()