malloc: extend malloc_usable_size() for contigmalloc

Extend malloc_usable_size() for contigmalloc; it seems the only
outside consumer is LinuxKPI ksize() which by itself has little to
no consumer either.

Sponsored by: The FreeBSD Foundation
Suggested by: jhb (see D46657)
Reviewed by: jhb, markj
Fixes: 9e6544dd6e02
Differential Revision: https://reviews.freebsd.org/D49571

(cherry picked from commit c5cf4b64f4a976f5c5d75d52816078def86bf3e4)