Add the beginnings of the fs/verity/ support layer, including the
Kconfig option and various helper functions for hashing.  To start, only
SHA-256 is supported, but other hash algorithms can easily be added.
Reviewed-by: NTheodore Ts'o <tytso@mit.edu>
Reviewed-by: NJaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: NEric Biggers <ebiggers@google.com>