btrfs: remove buffer heads form superblock handling

This patch series removes the use of buffer_heads from btrfs' super block read
and write paths. It also converts the integrity-checking code to only work
with pages and BIOs.

Compared to buffer heads, this gives us a leaner call path, as the
buffer_head code wraps around getting pages from the page-cache and adding
them to BIOs to submit.

The first patch removes buffer_heads from superblock reading.  The second
removes it from super_block writing and the subsequent patches remove the
buffer_heads from the integrity check code.

It's based on misc-next from Wednesday January 22, and doesn't show any
regressions in xfstests to the baseline.

Changes to v2:
- Removed patch #1 again
- Added Reviews from Josef
- Re-visited page locking, but not changes, it retains the same locking scheme
  the buffer_heads had
- Incroptorated comments from David regarding open-coding functions
- For more details see the idividual patches.


Changes to v1:
- Added patch #1
- Converted sb reading and integrity checking to use the page cache
- Added rationale behind the conversion to the commit messages.
- For more details see the idividual patches.


Johannes Thumshirn (5):
  btrfs: remove buffer heads from super block reading
  btrfs: remove use of buffer_heads from superblock writeout
  btrfs: remove btrfsic_submit_bh()
  btrfs: remove buffer_heads from btrfsic_process_written_block()
  btrfs: remove buffer_heads form superblock mirror integrity checking

 fs/btrfs/check-integrity.c | 218 +++++++++++--------------------------
 fs/btrfs/check-integrity.h |   2 -
 fs/btrfs/disk-io.c         | 200 +++++++++++++++++++++-------------
 fs/btrfs/disk-io.h         |   4 +-
 fs/btrfs/volumes.c         |  66 ++++++-----
 fs/btrfs/volumes.h         |   2 -
 6 files changed, 230 insertions(+), 262 deletions(-)

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2.24.1