nfs: fix NPE on door reboot

handle PoolPassiveIoFileMessage message comming from a pool
even if door did not initialize it. Happend on door restarts
during heavy IO load.

Acked-by: Paul Millar
Target: master
Require-book: no
Require-notes: no
(cherry picked from commit e4f8fc5c9528e40ca7e27083c6ae92a38d5a9de0)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>