mds: fix occasional dir rstat inconsistency between multi-MDSes

mds: fix occasional dir rstat inconsistency between multi-MDSes

Currently if file inode and its parent are processed on auth MDS,
its parent's rstat will be updated correctly, but won't be passed
to non-auth MDS. So if a newly-mounted ceph-fuse client connects
to non-auth MDS, this parent's rstat might be old.

The fix is to forward getattr request with CEPH_STAT_RSTAT mask
to auth MDS for latest rstat and client will always update its rstat
if reply is from auth MDS.

Signed-off-by: Zhi Zhang <zhangz.david@outlook.com>