EX-3144 pcc: revalidate the pointer after attach

EX-3144 pcc: revalidate the pointer after attach

We need refresh pointer again since the lock may
be released in pcc_try_readonly_open_attach.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I470358dfde525e08e7110e862b30b527e5db94fe
Reviewed-on: https://review.whamcloud.com/43662
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>