LU-17243 pcc: replace i_mtime with inode_get_mtime()

LU-17243 pcc: replace i_mtime with inode_get_mtime()

This patch replaces i_mtime with inode_get_mtime(), and
i_mtime.tv_sec with inode_get_mtime_sec() in pcc codes.

Test-Parameters: trivial testlist=sanity-pcc

Fixes: 3c586ca ("LU-17243 build: compatibility updates for kernel 6.6")
Change-Id: I756451ac9d38c3b434bb511b33d5d891b2b914ae
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54357
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>