ceph: fix snapshot directory timestamps

The .snap directory timestamps are kept at 0 (1970-01-01 00:00), which
isn't consistent with what the fuse client does.  This patch makes the
behaviour consistent, by setting these timestamps (atime, btime, ctime,
mtime) to those of the parent directory.

Cc: Marc Roos <M.Roos@f1-outsourcing.eu>
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>