This bug also affects stretch's kernel (package linux-image-4.9.0-4, version 4.9.51-1, as of 2017-09-28) in a very similar scenario that Andrew Chadwick reported (i.e. 20+ domain members on a cifs share via multiuser,sec=krb5 option).
Applying the patch (via test-patches) from the sid branch seems to restore the expected behavior. Axel Schäfer
