Currently btrfs handles checksum validation and repair in the end I/O
handler for the btrfs_bio.  This leads to a lot of duplicate code
plus issues with varying semantics or bugs, e.g.

- the until recently broken repair for compressed extents
- the fact that encoded reads validate the checksums but do not kick
  of read repair
- the inconsistent checking of the BTRFS_FS_STATE_NO_CSUMS flag

This commit revamps the checksum validation and repair code to instead
work below the btrfs_submit_bio interfaces.

In case of a checksum failure (or a plain old I/O error), the repair
is now kicked off before the upper level ->end_io handler is invoked.

Progress of an in-progress repair is tracked by a small structure
that is allocated using a mempool for each original bio with failed
sectors, which holds a reference to the original bio.   This new
structure is allocated using a mempool to guarantee forward progress
even under memory pressure.  The mempool will be replenished when
the repair completes, just as the mempools backing the bios.

There is one significant behavior change here:  If repair fails or
is impossible to start with, the whole bio will be failed to the
upper layer.  This is the behavior that all I/O submitters except
for buffered I/O already emulated in their end_io handler.  For
buffered I/O this now means that a large readahead request can
fail due to a single bad sector, but as readahead errors are ignored
the following readpage if the sector is actually accessed will
still be able to read.  This also matches the I/O failure handling
in other file systems.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>