fs-verity: move structs needed for file signing to UAPI header

Although it isn't used directly by the ioctls,
"struct fsverity_descriptor" is required by userspace programs that need
to compute fs-verity file digests in a standalone way.  Therefore
it's also needed to sign files in a standalone way.

Similarly, "struct fsverity_formatted_digest" (previously called
"struct fsverity_signed_digest" which was misleading) is also needed to
sign files if the built-in signature verification is being used.

Therefore, move these structs to the UAPI header.

While doing this, try to make it clear that the signature-related fields
in fsverity_descriptor aren't used in the file digest computation.

Acked-by: Luca Boccassi <luca.boccassi@microsoft.com>
Link: https://lore.kernel.org/r/20201113211918.71883-5-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

Documentation/filesystems/fsverity.rst

@@ -334,17 +334,13 @@ root hash as well as other fields such as the file size::
             __u8 hash_algorithm;    /* Merkle tree hash algorithm */
             __u8 log_blocksize;     /* log2 of size of data and tree blocks */
             __u8 salt_size;         /* size of salt in bytes; 0 if none */
-            __le32 sig_size;        /* must be 0 */
+            __le32 __reserved_0x04; /* must be 0 */
             __le64 data_size;       /* size of file the Merkle tree is built over */
             __u8 root_hash[64];     /* Merkle tree root hash */
             __u8 salt[32];          /* salt prepended to each hashed block */
             __u8 __reserved[144];   /* must be 0's */
     };
 
-Note that the ``sig_size`` field must be set to 0 for the purpose of
-computing the file measurement, even if a signature was provided (or
-will be provided) to `FS_IOC_ENABLE_VERITY`_.
-
 Built-in signature verification
 ===============================