pstore: gracefully handle NULL pstore_info functions

If a pstore backend doesn't want to support various portions of the
pstore interface, it can just leave those functions NULL instead of
creating no-op stubs.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/inode.c

@@ -80,7 +80,8 @@ static int pstore_unlink(struct inode *dir, struct dentry *dentry)
 {
 	struct pstore_private *p = dentry->d_inode->i_private;
 
-	p->psi->erase(p->type, p->id, p->psi);
+	if (p->psi->erase)
+		p->psi->erase(p->type, p->id, p->psi);
 
 	return simple_unlink(dir, dentry);
 }

fs/pstore/platform.c

@@ -207,8 +207,7 @@ void pstore_get_records(int quiet)
 		return;
 
 	mutex_lock(&psi->read_mutex);
-	rc = psi->open(psi);
-	if (rc)
+	if (psi->open && psi->open(psi))
 		goto out;
 
 	while ((size = psi->read(&id, &type, &time, &buf, psi)) > 0) {
@@ -219,7 +218,8 @@ void pstore_get_records(int quiet)
 		if (rc && (rc != -EEXIST || !quiet))
 			failed++;
 	}
-	psi->close(psi);
+	if (psi->close)
+		psi->close(psi);
 out:
 	mutex_unlock(&psi->read_mutex);