LU-17753 llite: ll_getattr_dentry(): protect access to times

LU-17753 llite: ll_getattr_dentry(): protect access to times

ll_merge_attr() updates inode's timestamps twice. Racing stat may get
intermediate values of timestamps.

Test to illustrate the issue is added.

ll_merge_attr() makes intermediate inode's timestamp update with
values from MDS under
ll_inode_info->lli_size_mutex. ll_getattr_dentry() should also use the
mutex to not return intermediate timestamps. This applies for regular
files only.

Test-Parameters: testlist=sanity env=ONLY=39u,ONLY_REPEAT=200
HPE-bug-id: LUS-12265
Signed-off-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Change-Id: Ic94040418da2a5cc5e41458dbf9d4c05244741ff
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54843
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>