nfsd: fix timestamp updates in CB_GETATTR

When updating the local timestamps from CB_GETATTR, the updated values
are not being properly vetted.

Compare the update times vs. the saved times in the delegation rather
than the current times in the inode. Also, ensure that the ctime is
properly vetted vs. its original value.

Fixes: 6ae30d6e ("nfsd: add support for delegated timestamps")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>