ext4: add fs-verity read support

Make ext4_mpage_readpages() verify data as it is read from fs-verity
files, using the helper functions from fs/verity/.

To be compatible with fscrypt, like in the corresponding f2fs patch this
required refactoring the decryption workflow into a generic "post-read
processing" workflow, which can do decryption, verification, or both.

Co-developed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
4 files changed