sunrpc: kstrtoul() can also return -ERANGE

Smatch complains that "num" can be uninitialized when kstrtoul() returns
-ERANGE.  It's true enough, but basically harmless in this case.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>