pstore/blk: Improve failure reporting

There was no feedback on bad registration attempts. Add details on the
failure cause.

Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/blk.c

@@ -114,8 +114,22 @@ static int __register_pstore_device(struct pstore_device_info *dev)
 
 	lockdep_assert_held(&pstore_blk_lock);
 
-	if (!dev || !dev->total_size || !dev->read || !dev->write)
+	if (!dev) {
+		pr_err("NULL device info\n");
 		return -EINVAL;
+	}
+	if (!dev->total_size) {
+		pr_err("zero sized device\n");
+		return -EINVAL;
+	}
+	if (!dev->read) {
+		pr_err("no read handler for device\n");
+		return -EINVAL;
+	}
+	if (!dev->write) {
+		pr_err("no write handler for device\n");
+		return -EINVAL;
+	}
 
 	/* someone already registered before */
 	if (pstore_zone_info)