fs-verity: fix all kerneldoc warnings

Fix all kerneldoc warnings in fs/verity/ and include/linux/fsverity.h.
Most of these were due to missing documentation for function parameters.

Detected with:

    scripts/kernel-doc -v -none fs/verity/*.{c,h} include/linux/fsverity.h

This cleanup makes it possible to check new patches for kerneldoc
warnings without having to filter out all the existing ones.

Link: https://lore.kernel.org/r/20200511192118.71427-2-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>