Header file changes for TSO

Via Drew.
warn_once()

To avoid massinve warning spam.
Fixes allocb's use of mem flags

_allocb was always waiting, even for iallocb.  Now iallocb can fail if
memory isn't available (as it was meant to), and allocb will block til
memory is available.
Block extra_data

Extends blocks to have an array of pointers to extra data.  The old
block buffer is now the 'main body'.  A block's data consists of the
main body, followed by the extra_data buffers, in order.

The extra data are currently kmalloc refcnt'd and are kfreed when the
block is freed.  Later, we'll want some way to handle other, probably
refcnt'd buffers.

Blocks are ideally kept in their scattered state, but most of our NIC
drivers do not know how to handle this yet.  For those, we linearize the
block before pushing it out devether.

Checksums, qdiscards, copies, clones, etc, all needed to be changed to
know about the new packet type.  There are also a lot of places where we
don't handle the new block types.  If you see any of the warnings, then
let me know.

Likewise, if you have weird networking failures, turn off
CONFIG_BLOCK_EXTRAS and see if that fixes it.  Regardless, let me know
the outcome.