fs-verity: add data verification hooks for ->readpages()

Add functions that verify data pages that have been read from a
fs-verity file, against that file's Merkle tree.  These will be called
from filesystems' ->readpage() and ->readpages() methods.

Since data verification can block, a workqueue is provided for these
methods to enqueue verification work from their bio completion callback.

See the "Verifying data" section of
Documentation/filesystems/fsverity.rst for more information.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/verity/init.c

@@ -41,7 +41,15 @@ static int __init fsverity_init(void)
 	if (err)
 		return err;
 
+	err = fsverity_init_workqueue();
+	if (err)
+		goto err_exit_info_cache;
+
 	pr_debug("Initialized fs-verity\n");
 	return 0;
+
+err_exit_info_cache:
+	fsverity_exit_info_cache();
+	return err;
 }
 late_initcall(fsverity_init)