ocfs2: Reset xattr value size after xa_cleanup_value_truncate().

In ocfs2_prepare_xattr_entry, if we fail to grow an existing value,
xa_cleanup_value_truncate() will leave the old entry in place.  Thus, we
reset its value size.  However, if we were allocating a new value, we
must not reset the value size or we will BUG().  This resolves
oss.oracle.com bug 1247.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>