ceph: properly handle granular statx requests

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/39258


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>