cephfs can benefit from statx. We can have the client just request caps
sufficient for the needed attributes and leave off the rest.

Also, recognize when AT_STATX_DONT_SYNC is set, and just scrape the
inode without doing any call in that case. Force a call to the MDS in
the event that AT_STATX_FORCE_SYNC is set.

Link: http://tracker.ceph.com/issues/39258Signed-off-by: NJeff Layton <jlayton@kernel.org>
Reviewed-by: N"Yan, Zheng" <zyan@redhat.com>
Reviewed-by: NDavid Howells <dhowells@redhat.com>
Reviewed-by: NSage Weil <sage@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>