kernel - Introduce lightweight buffers
* Summary:
The lightweight buffer (lwbuf) subsystem is effectively a reimplementation
of the sfbuf (sendfile buffers) implementation. It was designed to be
lighter weight than the sfbuf implementation when possible, on x86_64
we use the DMAP and the implementation is -very- simple. It was also
designed to be more SMP friendly.
* Replace all consumption of sfbuf with lwbuf
* Refactor sfbuf to act as an external refcount mechanism for sendfile(2),
this will probably go away eventually as well.
- [DB] sys/cpu/x86_64/misc/lwbuf.c