nfsv4: leave the synchronized block when disposing a client

As Joshua Bloch says in "Effective Java" -

"Invoking alien methods from synchronized regions has caused
many deadlocks in real systems". Now we are one of them.

Observed on CMS instance @ DESY.

Acked-by: Gerd Behrmann
Target: master, 0.9, 0.8
Require-book: no
Require-notes: no
(cherry picked from commit 06b9784ce462c767afe0d583df080d5c4aa7a6cc)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>