btrfs: stop parsing crc32c driver name

To determine whether the crc32c implementation is "fast", use
crc32_optimizations() instead of parsing the crypto_shash driver name.
This keeps the code working as intended after the driver name is changed
by the next commit.

Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/20250613183753.31864-2-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>