Introduce a helper function for filesystems to call when processing
->setattr() on a possibly-encrypted inode.  It handles enforcing that an
encrypted file can only be truncated if its encryption key is available.
Acked-by: NDave Chinner <dchinner@redhat.com>
Signed-off-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>