ext4: simplify fast-commit CRC calculation

Instead of checksumming each field as it is added to the block, just
checksum each block before it is written.  This is simpler, and also
much more efficient.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20221106224841.279231-8-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>