btrfs: make sure all btrfs_bio::end_io are called in task context

btrfs: make sure all btrfs_bio::end_io are called in task context

[BACKGROUND]
Btrfs has a lot of different bi_end_io functions, to handle different
raid profiles. But they introduced a lot of different contexts for
btrfs_bio::end_io() calls:

- Simple read bios
  Run in task context, backed by either endio_meta_workers or
  endio_workers.

- Simple write bios
  Run in IRQ context.

- RAID56 write or rebuild bios
  Run in task context, backed by rmw_workers.

- Mirrored write bios
  Run in irq context.

This is inconsistent, and contributes to the number of workqueues used
in btrfs.

[ENHANCEMENT]
Make all the above bios call their btrfs_bio::end_io() in task context,
backed by either endio_meta_workers for metadata, or endio_workers for
data.

For simple write bios, merge the handling into simple_end_io_work().

For mirrored write bios, it will be a little more complex, since both
the original or the cloned bios can run the final btrfs_bio::end_io().

Here we make sure the cloned bios are using btrfs_bioset, to reuse the
end_io_work, and run both original and cloned work inside the workqueue.

Add extra ASSERT()s to make sure btrfs_bio_end_io() is running in task
context.

This not only unifies the context for btrfs_bio::end_io() functions, but
also opens a new door for further btrfs_bio::end_io() related cleanups.

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