flexfiles/pNFS: update stats on NFS4ERR_DELAY for v4.1 DSes

On NFS4ERR_DELAY nfs slient updates its stats, but misses for
flexfiles v4.1 DSes.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>