btrfs: switch to library APIs for checksums

btrfs: switch to library APIs for checksums

Make btrfs use the library APIs instead of crypto_shash, for all
checksum computations.  This has many benefits:

- Allows future checksum types, e.g. XXH3 or CRC64, to be more easily
  supported.  Only a library API will be needed, not crypto_shash too.

- Eliminates the overhead of the generic crypto layer, including an
  indirect call for every function call and other API overhead.  A
  microbenchmark of btrfs_check_read_bio() with crc32c checksums shows a
  speedup from 658 cycles to 608 cycles per 4096-byte block.

- Decreases the stack usage of btrfs by reducing the size of checksum
  contexts from 384 bytes to 240 bytes, and by eliminating the need for
  some functions to declare a checksum context at all.

- Increases reliability.  The library functions always succeed and
  return void.  In contrast, crypto_shash can fail and return errors.
  Also, the library functions are guaranteed to be available when btrfs
  is loaded; there's no longer any need to use module softdeps to try to
  work around the crypto modules sometimes not being loaded.

- Fixes a bug where blake2b checksums didn't work on kernels booted with
  fips=1.  Since btrfs checksums are for integrity only, it's fine for
  them to use non-FIPS-approved algorithms.

Note that with having to handle 4 algorithms instead of just 1-2, this
commit does result in a slightly positive diffstat.  That being said,
this wouldn't have been the case if btrfs had actually checked for
errors from crypto_shash, which technically it should have been doing.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>