ipfilter: Cast uintmax_t values to size_t when adding to a pointer.

[jhb]

Summary

GCC warns about the mismatched sizes on 32-bit platforms where
uintmax_t is larger in size than a pointer.

[cy]

[imp]

assuming that you know that these can never be truncated, and that truncation won't cause a problem.

[jhb]

In D36753#834438, @imp wrote:

assuming that you know that these can never be truncated, and that truncation won't cause a problem.

If you have an offset bigger than size_t, you don't have a valid pointer (or you've wrapped around anyway). In practice arg2 are fairly small values that are offsets of fields in structures.