client: getattr before read on ceph.* xattrs

client: getattr before read on ceph.* xattrs

Previously we were returning values for quota, layout
xattrs without any kind of update -- the user just got
whatever happened to be in our cache.

Clearly this extra round trip has a cost, but reads of
these xattrs are fairly rare, happening on admin
intervention rather than in normal operation.

Fixes: http://tracker.ceph.com/issues/17939
Signed-off-by: John Spray <john.spray@redhat.com>