fsverity: generate and store zero-block hash

fsverity: generate and store zero-block hash

Compute the hash of one filesystem block's worth of zeros. A filesystem
implementation can decide to elide merkle tree blocks containing only
this hash and synthesize the contents at read time.

Let's pretend that there's a file containing 131 data block and whose
merkle tree looks roughly like this:

root
 +--leaf0
 |   +--data0
 |   +--data1
 |   +--...
 |   `--data128
 `--leaf1
     +--data129
     +--data130
     `--data131

If data[0-128] are sparse holes, then leaf0 will contain a repeating
sequence of @zero_digest.  Therefore, leaf0 need not be written to disk
because its contents can be synthesized.

A subsequent xfs patch will use this to reduce the size of the merkle
tree when dealing with sparse gold master disk images and the like.

Note that this works only on the first-level (data holes). fsverity
doesn't store/generate zero_digest for any higher levels.

Add a helper to pre-fill folio with hashes of empty blocks. This will be
used by iomap to synthesize blocks full of zero hashes on the fly.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Acked-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
Link: https://patch.msgid.link/20260520123722.405752-5-aalbersh@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>