DIRCACHE_MIN_MEM_MB does not stringify on FreeBSD, we should use %d instead

A FreeBSD relay operator reports the message:

[WARN] Being a directory cache (default) with less than DIRCACHE_MIN_MB_BANDWIDTH MB of memory is not recommended and may consume most of the available resources, consider disabling this functionality by setting the DirCache option to 0

So we should print the macro value the same way we do every other value, using "%d" in the string, and passing it as an argument.

(This macro changed name in #20684 (moved).)