mds: set new inode's xattr version to 1

set new inode's xattr version to 1 even if it has no xattr. This allow
client to differentiate no xattr in inode from MDS skips sending xattr
to client (because MDS think client already has uptodate xattr).

Signed-off-by: Yan, Zheng <zyan@redhat.com>
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>
Merge pull request #2503 from ceph/wip-xattr-fix

Wip xattr fix