btrfs: add member for a specific checksum driver

Currently all the checksum algorithms generate a fixed size digest size
and we use it.  The on-disk format can hold up to BTRFS_CSUM_SIZE bytes
and BLAKE2b produces digest of 512 bits by default. We can't do that and
will use the blake2b-256, this needs to be passed to the crypto API.

Separate that from the base algorithm name and add a member to request
specific driver, in this case with the digest size.

The only place that uses the driver name is the crypto API setup.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -32,6 +32,7 @@ static void del_ptr(struct btrfs_root *root, struct btrfs_path *path,
 static const struct btrfs_csums {
 	u16		size;
 	const char	*name;
+	const char	*driver;
 } btrfs_csums[] = {
 	[BTRFS_CSUM_TYPE_CRC32] = { .size = 4, .name = "crc32c" },
 	[BTRFS_CSUM_TYPE_XXHASH] = { .size = 8, .name = "xxhash64" },
@@ -53,6 +54,17 @@ const char *btrfs_super_csum_name(u16 csum_type)
 	return btrfs_csums[csum_type].name;
 }
 
+/*
+ * Return driver name if defined, otherwise the name that's also a valid driver
+ * name
+ */
+const char *btrfs_super_csum_driver(u16 csum_type)
+{
+	/* csum type is validated at mount time */
+	return btrfs_csums[csum_type].driver ?:
+		btrfs_csums[csum_type].name;
+}
+
 size_t __const btrfs_get_num_csums(void)
 {
 	return ARRAY_SIZE(btrfs_csums);