Currently the checksum of compressed extents is verified based on the
compressed data and the lower btrfs_bio, but the actual repair process
is driven by end_bio_extent_readpage on the upper btrfs_bio for the
decompressed data.

This has a bunch of issues, including not being able to properly
communicate the failed mirror up in case that the I/O submission got
preempted, a general loss of if an error was an I/O error or a checksum
verification failure, but most importantly that this design causes
btrfs_clean_io_failure to eventually write back the uncompressed good
data onto the disk sectors that are supposed to contain compressed data.

Fix this by moving the repair to the lower btrfs_bio.  To do so, a fair
amount of code has to be reshuffled:

 a) the lower btrfs_bio now needs a valid csum pointer.  The easiest way
    to achieve that is to pass NULL btrfs_lookup_bio_sums and just use
    the btrfs_bio management of csums.  For a compressed_bio that is
    split into multiple btrfs_bios this means additional memory
    allocations, but the code becomes a lot more regular.
 b) checksum verification now runs directly on the lower btrfs_bio instead
    of the compressed_bio.  This actually nicely simplifies the end I/O
    processing.
 c) btrfs_repair_one_sector can't just look up the logical address for
    the file offset any more, as there is no corresponding relative
    offsets that apply to the file offset and the logic address for
    compressed extents.  Instead require that the saved bvec_iter in the
    btrfs_bio is filled out for all read bios and use that, which again
    removes a fair amount of code.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NDavid Sterba <dsterba@suse.com>