ceph: carry snapshot creation time with inodes

MDS InodeStat v3 wire structures include a trailing snapshot creation
time member. Unmarshall this and retain it for a future vxattr.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>