tcp_bbr: avoid gcc -Werror=pointer-to-int-cast on 32-bit arch

[rlibby]

Test Plan

make buildkernel on amd64 and i386, examine warnings

[tuexen]

Why is an uint64_t not good enough to hold a pointer? Aren't pointers 32-bit or 64-bit on all architectures FreeBSD supports?

[rlibby]

In D45751#1043496, @tuexen wrote:

Why is an uint64_t not good enough to hold a pointer? Aren't pointers 32-bit or 64-bit on all architectures FreeBSD supports?

Yeah, uint64_t is always wide enough. gcc's issue on 32-bit arch is that it doesn't like to see a 32-bit pointer cast directly to a 64-bit integer. It issues -Wpointer-to-int-cast, which is an error with our flags. This patch works around that by first casting to an integer type that is the same width as a pointer. From there there is no complaint about the conversion to uint64_t. The patch is intended to be a functional nop, for both 32-bit and 64-bit arch.

Same deal in D45752. Related issue in reverse in D45750.