LU-17948 llite: replace i_mtime.tv_sec with inode_get_mtime_sec()

LU-17948 llite: replace i_mtime.tv_sec with inode_get_mtime_sec()

This patch replaces i_mtime.tv_sec with inode_get_mtime_sec() to
fix the following build failure:

lustre/llite/pcc.c:1691:32: error:
'struct inode' has no member named 'i_mtime'; did you mean '__i_mtime'?
 1691 |         item.pm_mtime = inode->i_mtime.tv_sec;
      |                                ^~~~~~~
      |                                __i_mtime

Test-Parameters: trivial testlist=sanity-pcc

Change-Id: Iaed264c32be3d48039c5350ebd306f4fc3ef5eb9
Fixes: 3835f4d ("LU-13881 pcc: comparator support for PCC rules")
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55416
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>