UPSTREAM: 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>
Bug: 186190934
Test: With rest of series and ABI update incrementalfs.ko loads
(cherry picked from commit bde493349025ca0559e2fff88592935af3b8df19)
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I41007ebc9183be14e07e51486c0e6030b5aad187