EX-3764 pcc: avoid panic in asynchronous attach thread

EX-3764 pcc: avoid panic in asynchronous attach thread

When PCC attach a file asynchronously, it wrongly uses
@pccx->pccx_file after put the file in @pcc_attach_context_free().
This may casue panic in pcc_readonly_attach_thread().
This patch fixes it by using @pccx->pccx_file before put it.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Iaa93403e4db7497923033d327e689627790fa6a0
Reviewed-on: https://review.whamcloud.com/44881
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>