pstore: Improve error reporting in case of backend overlap

The pstore infrastructure supports one single backend at a time;
trying to load a another backend causes an error and displays a
message, introduced on commit 0d7cd09a3d ("pstore: Improve
register_pstore() error reporting").

Happens that this message is not really clear about the situation,
also the current error returned (-EPERM) isn't accurate, whereas
-EBUSY makes more sense. We have another place in the code that
relies in the -EBUSY return for a similar check.

So, make it consistent here by returning -EBUSY and using a
similar message in both scenarios.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221006224212.569555-2-gpiccoli@igalia.com

fs/pstore/platform.c

@@ -562,8 +562,9 @@ out:
 int pstore_register(struct pstore_info *psi)
 {
 	if (backend && strcmp(backend, psi->name)) {
-		pr_warn("ignoring unexpected backend '%s'\n", psi->name);
-		return -EPERM;
+		pr_warn("backend '%s' already in use: ignoring '%s'\n",
+			backend, psi->name);
+		return -EBUSY;
 	}
 
 	/* Sanity check flags. */