EX-2872 pcc: mtime rule for 'lctl pcc add'

EX-2872 pcc: mtime rule for 'lctl pcc add'

Add an "mtime>N" rule to allow skipping files for PCC-RO auto-attach
if they were created or modified more than N seconds ago.  Otherwise,
it may be that files are added to the PCC cache before they finished
writing, or if they will be modified again quickly after creation.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Ibb99bff5b483717ae6e5b83f82f1bcd86c3ebbe5
Reviewed-on: https://review.whamcloud.com/42122
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>