dummynet: Fix socket option length validation for IP_DUMMYNET3

[markj]

Summary

The socket option handler tries to ensure that the option length is no
larger than some reasonable maximum, and no smaller than sizeof(struct
dn_id). But the loaded option length is stored in an int, which is
converted to an unsigned integer for the comparison with a size_t, so
negative values are not caught and get passed to malloc().

Use a cast to enforce the intended comparison. Found by code
inspection.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

[kp]

Wouldn't it make more sense to change l to be a size_t? sopt->sopt_valsize is a size_t, and the arguments to sootcopyin() are also size_t.
It's only do_config() which takes an int, and that may want to be changed to take a size_t as well.

[markj]

In D33133#748895, @kp wrote:

Wouldn't it make more sense to change l to be a size_t? sopt->sopt_valsize is a size_t, and the arguments to sootcopyin() are also size_t.
It's only do_config() which takes an int, and that may want to be changed to take a size_t as well.

Yeah, that's the right solution.

[markj]

Make do_config() take a size_t instead.