Fix build on non-64bit architectures

[lwhsu]

Summary

r328351 seems breaks the architectures where size_t is not unsigned long:

https://ci.freebsd.org/job/FreeBSD-head-mips-build/95/console

00:06:35.591 --- kern_malloc.o ---
00:06:35.591 cc1: warnings being treated as errors
00:06:35.591 /usr/src/sys/kern/kern_malloc.c: In function 'malloc':
00:06:35.591 /usr/src/sys/kern/kern_malloc.c:532: warning: passing argument 2 of 'malloc_dbg' from incompatible pointer type
00:06:35.592 /usr/src/sys/kern/kern_malloc.c: In function 'malloc_domain':
00:06:35.592 /usr/src/sys/kern/kern_malloc.c:582: warning: passing argument 2 of 'malloc_dbg' from incompatible pointer type

BTW, I see more places in this files use unsigned long as size or alignment, should they also be changed?

[cem]

First: Sure, this change seems good.

Second: It's not clear to me -- why is size_t not unsigned long on MIPS? Or is this a long vs long long (but both are 64 bits) type complaint?

Third: Yes, John would like many more of those unsigned longs to become size_t. I'm on board. If this is what is needed to unbreak the build, though, there's no need to do that other stuff at the same time.

[lwhsu]

Thanks, I just wanted to fix the build first.

For the second, I am not sure, either. I guess the compiler just complains about types are different.

For the third, good to know that, I also don't want to do these change at the same time.