Siplify the code using the extended contigmalloc(9) which means we
(a) no longer have to remember our allocation size, and (b) we can
call free(9) independent of the allocator (malloc or contigmalloc).
This will likely also allow us to make the TUNABLE a SYSCTL in the
future, allow drivers to change it on the fly (at least lowering the
limit) and with that removing the need for user adjustments.
Sponsored by: The FreeBSD Foundation
MFC after: 3 days