We can only reset ->l_ast_data if it's NULL; otherwise, returning an
error if ->l_ast_data is trying to be changed to a different value.
Change-Id: I8f999a98101daa2c465256792285582f581909c5
Signed-off-by: Jinshan Xiong <jay@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1466
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>