LU-10499 pcc: keep mtime unchange when attach file into PCC

LU-10499 pcc: keep mtime unchange when attach file into PCC

Modifying the timestamps of the files for the attach will cause
problems for the cache manager, since all files will appear new
at the time they are imported into the cache.
And that may also confuse applications if the file mtime has
changed just because of attach.

In this patch, it keeps the file mtime unchanged when attach it
into PCC.

EX-bug-id: EX-2859
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I26f0c19c7b6cc1af0d62c192931d0042c9614993
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54424
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: jsimmons <jsimmons@infradead.org>