tst-zfs-mount.so: Disable reference count check

Commit 951cca4 ("Revert "vfs/zfs: Sync vnode and znode refcounts")
forgot to disable ZFS reference count test case.

Reported-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>