nfs4: fix lookupp on the root of the tree

fixes NPE triggered by pynfs test suite.

Acked-by: Pail Millar
Target: master
(cherry picked from commit 44e4d54a6a89e0caf61a82b360a61199736c2150)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
nfs4: fix handling of illegal operations

make the logic simpler and fix NPE.
Triggered by pynfs testsuite

Acked-by: Paul Millar
Target: master
(cherry picked from commit 0222c7d8395d6982e6ad81feb7fc5f9097a63666)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>