NFSv4: Don't error if we handled it in nfs4_recovery_handle_error

If we handled an error condition, then nfs4_recovery_handle_error should
return '0' so that the state recovery thread can continue.
Also ensure that nfs4_check_lease() continues to abort if we haven't got
any credentials by having it return ENOKEY (which is not handled).

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