ceph: add ceph.snap.btime vxattr

The ceph.snap.btime virtual xattr provides the snapshot creation (birth)
time in $secs.$nsecs format.

Link: https://tracker.ceph.com/issues/38838


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