mds: set xattr_version to 0 if replay doesn't contain xattrs

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #2030 from ceph/wip-xattr-fix

mds: set xattr_version to 0 if replay doesn't contain xattrs

Reviewed-by: Sage Weil <sage@redhat.com>