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>

drivers/firmware/efivars.c

@@ -1076,7 +1076,7 @@ static int efivarfs_fill_super(struct super_block *sb, void *data, int silent)
 
 		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);
 	}