etc/systemd/zfs-mount-generator: serialise

The wins for a relatively normal workload are rather slim:
	real	0.02119s/0.00985s=2.15029x
	user	0.02130s/0.00346s=6.15560x
	sys	0.03858s/0.00643s=6.00062x

	wall-total	0.014518s/0.005925s=2.45009x
	wall-init	0.014518s/0.002457s=5.90684x
	wall-real	0.014518s/0.003467s=4.18668x

But this is a big win on machines with a lot of datasets and expensive
forks.

For example, the gain on a VM on my work laptop with 900+ legacy-mount
Docker datasets, the original gains from the C rewrite were
only five-fold:
	real    0.516s/0.102s=5.05882x
	user    0.237s/0.143s=1.65734x
	sys     0.287s/0.100s=2.87x

And this serial variant gains this back there as well:
	real    0.102s/0.008s=12.75x
	user    0.143s/0.007s=20.42857
	sys     0.100s/0.001s=100x

	wall-total	0.09717s/0.00319s=30.40255x
	wall-init	0.00203s/0.00200s=1.015941x
	wall-real	0.09513s/0.00118s=80.02043x

For a total of
	real    0.516s/0.008s=64.5x
	user    0.237s/0.007s=33.85714x
	sys     0.287s/0.001s=287x

Suggested-by: Richard Laager <[email protected]>
Signed-off-by: Ahelenia Ziemiańska <[email protected]>