NFSv4: The state manager shouldn't exit on errors that were handled

nfs4_recovery_handle_error() will correctly handle errors such as
NFS4ERR_CB_PATH_DOWN, however because they are still passed back to the
main loop in nfs4_state_manager(), they can cause the latter to exit
prematurely.

Fix this by letting nfs4_recovery_handle_error() change the error value in
cases where there is no action required by the caller.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>