pstore: Improve register_pstore() error reporting

Uncommon errors are better to get reported to dmesg so developers can
more easily figure out why pstore is unhappy with a backend attempting
to register.

Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/platform.c

@@ -673,11 +673,15 @@ int pstore_register(struct pstore_info *psi)
 {
 	struct module *owner = psi->owner;
 
-	if (backend && strcmp(backend, psi->name))
+	if (backend && strcmp(backend, psi->name)) {
+		pr_warn("ignoring unexpected backend '%s'\n", psi->name);
 		return -EPERM;
+	}
 
 	spin_lock(&pstore_lock);
 	if (psinfo) {
+		pr_warn("backend '%s' already loaded: ignoring '%s'\n",
+			psinfo->name, psi->name);
 		spin_unlock(&pstore_lock);
 		return -EBUSY;
 	}