The memory-limit is an unsigned integer. It is ugly (if not wrong) to compare unsigned
values with "-1". When comparing with the default value we must also use an u32 type.
Instead add a define NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET.

Note that like iproute2 we treat NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET
to indicate to not set TCA_FQ_CODEL_MEMORY_LIMIT in RTM_NEWQDISC. This
special value is entirely internal to NetworkManager (or iproute2) and
kernel will then choose a default memory limit (of 32MB). So setting
NM_PLATFORM_FQ_CODEL_MEMORY_LIMIT_UNSET means to leave it to kernel to
choose a value (which then chooses 32MB).

See kernel's net/sched/sch_fq_codel.c:

    static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt,
                             struct netlink_ext_ack *extack)
    {
    ...
            q->memory_limit = 32 << 20; /* 32 MBytes */

    static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
                               struct netlink_ext_ack *extack)
    ...
            if (tb[TCA_FQ_CODEL_MEMORY_LIMIT])
                    q->memory_limit = min(1U << 31, nla_get_u32(tb[TCA_FQ_CODEL_MEMORY_LIMIT]));

Note that not having zero as default value is problematic. In fields like
"NMPlatformIP4Route.table_coerced" and "NMPlatformRoutingRule.suppress_prefixlen_inverse"
we avoid this problem by storing a coerced value in the structure so that zero is still
the default. We don't do that here for memory-limit, so the caller must always explicitly
set the value.