ceph: choose auth MDS for getxattr with the Xs caps

ceph: choose auth MDS for getxattr with the Xs caps

And for the 'Xs' caps for getxattr we will also choose the auth MDS,
because the MDS side code is buggy due to setxattr won't notify the
replica MDSes when the values changed and the replica MDS will return
the old values. Though we will fix it in MDS code, but this still
makes sense for old ceph.

URL: https://tracker.ceph.com/issues/55331
Signed-off-by: Xiubo Li <xiubli@redhat.com>