btrfs: introduce btrfs_bio::async_csum

[ENHANCEMENT]
Btrfs currently calculates data checksums then submits the bio.

But after commit 968f19c5b1b7 ("btrfs: always fallback to buffered write
if the inode requires checksum"), any writes with data checksum will
fallback to buffered IO, meaning the content will not change during
writeback.

This means we're safe to calculate the data checksum and submit the bio
in parallel, and only need the following new behavior:

- Wait the csum generation to finish before calling btrfs_bio::end_io()
  Or this can lead to use-after-free for the csum generation worker.

- Save the current bi_iter for csum_one_bio()
  As the submission part can advance btrfs_bio::bio.bi_iter, if not
  saved csum_one_bio() may got an empty bi_iter and do not generate any
  checksum.

  Unfortunately this means we have to increase the size of btrfs_bio for
  16 bytes, but this is still acceptable.

As usual, such new feature is hidden behind the experimental flag.

[THEORETIC ANALYZE]
Consider the following theoretic hardware performance, which should be
more or less close to modern mainstream hardware:

	Memory bandwidth:	50GiB/s
	CRC32C bandwidth:	45GiB/s
	SSD bandwidth:		8GiB/s

Then write bandwidth with data checksum before the patch is:

	1 / ( 1 / 50 + 1 / 45 + 1 / 8) = 5.98 GiB/s

After the patch, the bandwidth is:

	1 / ( 1 / 50 + max( 1 / 45 + 1 / 8)) = 6.90 GiB/s

The difference is 15.32% improvement.

[REAL WORLD BENCHMARK]
I'm using a Zen5 (HX 370) as the host, the VM has 4GiB memory, 10 vCPUs, the
storage is backed by a PCIe gen3 x4 NVMe.

The test is a direct IO write, with 1MiB block size, write 7GiB data
into a btrfs mount with data checksum. Thus the direct write will
fallback to buffered one:

Vanilla Datasum:	1619.97 GiB/s
Patched Datasum:	1792.26 GiB/s
Diff			+10.6 %

In my case, the bottleneck is the storage, thus the improvement is not
reaching the theoretic one, but still some observable improvement.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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