mds: return xattrs when client explictly requests xattrs

Signed-off-by: Yan, Zheng <zyan@redhat.com>
client: request xattrs if xattr_version is 0

Following sequence of events can happen.
 - Client releases an inode, queues cap release message.
 - A 'lookup' reply brings the same inode back, but the reply
   doesn't contain xattrs because MDS didn't receive the cap release
   message and thought client already has up-to-data xattrs.

The fix is force sending a getattr request to MDS if xattrs_version is 0.
The getattr mask is set to CEPH_STAT_CAP_XATTR, so MDS knows client does
not have xattr.

Signed-off-by: Yan, Zheng <zyan@redhat.com>