ceph: acquire Fs caps when getting dir stats

We only update the inode's dirstats when we have Fs caps from the MDS.

Declare a new VXATTR_FLAG_DIRSTAT that we set on all dirstats, and have
the vxattr handling code acquire those caps when it's set.

URL: https://tracker.ceph.com/issues/48104


Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Patrick Donnelly <pdonnell@redhat.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>