fs-verity: support builtin file signatures

To meet some users' needs, add optional support for having fs-verity
handle a portion of the authentication policy in the kernel.  An
".fs-verity" keyring is created to which X.509 certificates can be
added; then a sysctl 'fs.verity.require_signatures' can be set to cause
the kernel to enforce that all fs-verity files contain a signature of
their file measurement by a key in this keyring.

See the "Built-in signature verification" section of
Documentation/filesystems/fsverity.rst for the full documentation.

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

fs/verity/verify.c

@@ -273,3 +273,9 @@ int __init fsverity_init_workqueue(void)
 		return -ENOMEM;
 	return 0;
 }
+
+void __init fsverity_exit_workqueue(void)
+{
+	destroy_workqueue(fsverity_read_workqueue);
+	fsverity_read_workqueue = NULL;
+}