client: ensure we take Fs caps when querying dirstat vxattrs

client: ensure we take Fs caps when querying dirstat vxattrs

Dirstats values are only updated when Fs caps are held, so ensure that
we request those caps in the _getattr call when querying for them.

Drop the old XATTR_NAME_CEPH helper and rename XATTR_NAME_CEPH2 to
XATTR_NAME_CEPH. Add a new VXATTR_DIRSTAT field and add that to all of
the dirstats vxattrs.

Fixes: https://tracker.ceph.com/issues/48313
Signed-off-by: Jeff Layton <jlayton@redhat.com>
(cherry picked from commit 3db582b1416dc170a0e25c957166d7bdc58e4c67)

Conflicts:
	src/client/Client.cc
	  - keep hidden because we still need it in nautilus
	  - drop some code because we did not introduce the
            following features: ceph.mirror.info and rsnaps