efivarfs: Use sizeof() instead of magic number

Instead of adding a magic 4 to the variable size, use sizeof() to make
it explicitly clear what the quantity represents (the variable's
attributes).

CC: Jeremy Kerr <jeremy.kerr@canonical.com>
Cc: Chun-Yi Lee <joeyli.kernel@gmail.com>
Cc: Andy Whitcroft <apw@canonical.com>
Reported-by: Lingzhu Xiang <lxiang@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
diff --git a/drivers/firmware/efivars.c b/drivers/firmware/efivars.c
index f5596db..371c441 100644
--- a/drivers/firmware/efivars.c
+++ b/drivers/firmware/efivars.c
@@ -1076,7 +1076,7 @@
 
 		mutex_lock(&inode->i_mutex);
 		inode->i_private = entry;
-		i_size_write(inode, size+4);
+		i_size_write(inode, size + sizeof(entry->var.Attributes));
 		mutex_unlock(&inode->i_mutex);
 		d_add(dentry, inode);
 	}