libcrc32c: Add crc32c_impl function

libcrc32c: Add crc32c_impl function

This function returns a string with the currently in-use implementation
of the crc32c algorithm, i.e crc32c-generic (for unoptimised, generic
implementation) or crc32c-intel for the sse optimised version. This
will be used by btrfs.

Signed-off-by: Nikolay Borisov <[email protected]>
Acked-by: Herbert Xu <[email protected]>
[ use crypto_shash_driver_name as suggested by Herbert ]
Signed-off-by: David Sterba <[email protected]>