nfs4: fix IO on special files

regression introduced in 93ec46a3a1be602c0e47d54abd8b0cb12ebb730f

Acked-by: Albert Rossi
Target: master, 2.7
Require-book: no
Require-notes: no
(cherry picked from commit 4da6c34736522a7d52484d1c477605095fe8d91f)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>