um: vector: Fix memory leak in vector_config

If the return value of the uml_parse_vector_ifspec function is NULL,
we should call kfree(params) to prevent memory leak.

Fixes: 49da7e64 ("High Performance UML Vector Network Driver")
Signed-off-by: Xiang Yang <xiangyang3@huawei.com>
Acked-By: Anton Ivanov <anton.ivanov@kot-begemot.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>