efi, pstore: Read data from variable store before memcpy()

Seiji reported getting empty dmesg-* files, because the data was never
actually read in efi_pstore_read_func(), and so the memcpy() was copying
garbage data.

This patch necessitated adding __efivar_entry_get() which is callable
between efivar_entry_iter_{begin,end}(). We can also delete
__efivar_entry_size() because efi_pstore_read_func() was the only
caller.

Reported-by: Seiji Aguchi <seiji.aguchi@hds.com>
Tested-by: Seiji Aguchi <seiji.aguchi@hds.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

drivers/firmware/efi/efi-pstore.c

@@ -73,7 +73,11 @@ static int efi_pstore_read_func(struct efivar_entry *entry, void *data)
 	} else
 		return 0;
 
-	__efivar_entry_size(entry, &size);
+	entry->var.DataSize = 1024;
+	__efivar_entry_get(entry, &entry->var.Attributes,
+			   &entry->var.DataSize, entry->var.Data);
+	size = entry->var.DataSize;
+
 	*cb_data->buf = kmalloc(size, GFP_KERNEL);
 	if (*cb_data->buf == NULL)
 		return -ENOMEM;