fs-verity: support reading Merkle tree with ioctl

Add support for FS_VERITY_METADATA_TYPE_MERKLE_TREE to
FS_IOC_READ_VERITY_METADATA.  This allows a userspace server program to
retrieve the Merkle tree of a verity file for serving to a client which
implements fs-verity compatible verification.  See the patch which
introduced FS_IOC_READ_VERITY_METADATA for more details.

This has been tested using a new xfstest which calls this ioctl via a
new subcommand for the 'fsverity' program from fsverity-utils.

Link: https://lore.kernel.org/r/20210115181819.34732-5-ebiggers@kernel.org
Reviewed-by: Victor Hsieh <victorhsieh@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

include/uapi/linux/fsverity.h

@@ -83,6 +83,8 @@ struct fsverity_formatted_digest {
 	__u8 digest[];
 };
 
+#define FS_VERITY_METADATA_TYPE_MERKLE_TREE	1
+
 struct fsverity_read_metadata_arg {
 	__u64 metadata_type;
 	__u64 offset;