xfs: drop async cache flushes from CIL commits.

Jan Kara reported a performance regression in dbench that he
bisected down to commit bad77c37 ("xfs: CIL checkpoint
flushes caches unconditionally").

Whilst developing the journal flush/fua optimisations this cache was
part of, it appeared to made a significant difference to
performance. However, now that this patchset has settled and all the
correctness issues fixed, there does not appear to be any
significant performance benefit to asynchronous cache flushes.

In fact, the opposite is true on some storage types and workloads,
where additional cache flushes that can occur from fsync heavy
workloads have measurable and significant impact on overall
throughput.

Local dbench testing shows little difference on dbench runs with
sync vs async cache flushes on either fast or slow SSD storage, and
no difference in streaming concurrent async transaction workloads
like fs-mark.

Fast NVME storage.

From `dbench -t 30`, CIL scale:

clients		async			sync
		BW	Latency		BW	Latency
1		 935.18   0.855		 915.64   0.903
8		2404.51   6.873		2341.77   6.511
16		3003.42   6.460		2931.57   6.529
32		3697.23   7.939		3596.28   7.894
128		7237.43  15.495		7217.74  11.588
512		5079.24  90.587		5167.08  95.822

fsmark, 32 threads, create w/ 64 byte xattr w/32k logbsize

	create		chown		unlink
async   1m41s		1m16s		2m03s
sync	1m40s		1m19s		1m54s

Slower SATA SSD storage:

From `dbench -t 30`, CIL scale:

clients		async			sync
		BW	Latency		BW	Latency
1		  78.59  15.792		  83.78  10.729
8		 367.88  92.067		 404.63  59.943
16		 564.51  72.524		 602.71  76.089
32		 831.66 105.984		 870.26 110.482
128		1659.76 102.969		1624.73  91.356
512		2135.91 223.054		2603.07 161.160

fsmark, 16 threads, create w/32k logbsize

	create		unlink
async   5m06s		4m15s
sync	5m00s		4m22s

And on Jan's test machine:

                   5.18-rc8-vanilla       5.18-rc8-patched
Amean     1        71.22 (   0.00%)       64.94 *   8.81%*
Amean     2        93.03 (   0.00%)       84.80 *   8.85%*
Amean     4       150.54 (   0.00%)      137.51 *   8.66%*
Amean     8       252.53 (   0.00%)      242.24 *   4.08%*
Amean     16      454.13 (   0.00%)      439.08 *   3.31%*
Amean     32      835.24 (   0.00%)      829.74 *   0.66%*
Amean     64     1740.59 (   0.00%)     1686.73 *   3.09%*

Performance and cache flush behaviour is restored to pre-regression
levels.

As such, we can now consider the async cache flush mechanism an
unnecessary exercise in premature optimisation and hence we can
now remove it and the infrastructure it requires completely.

Fixes: bad77c37 ("xfs: CIL checkpoint flushes caches unconditionally")
Reported-and-tested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>