ext2fs: Multi-block allocator with extent tree support and allocation lifecycle state machine

[pfg]

Summary

The idea was to have an allocator that uses better the locality information and would be better suited for extents and it ended being similar to what linux calls a multiblock allocator in a paper from 2005.
I have never seem the code and I doubt the specific implementation has any similarity.

Implement ext2_alloc_run() — a multi-block physical allocator that
reserves a contiguous physical block run and returns it via a
context-based state machine:

ALLOCATED -> MAPPED -> PUBLISHED -> ROLLED_BACK

ext2_alloc_run() enforces the reserved-block policy using struct
ucred *cred (NOCRED panics under INVARIANTS). Callers include
ext2_alloc() and ext4_new_blocks() for block allocation.

ext4_ext_split() uses scalar ext2_rollback_unpublished() for metadata
block cleanup (no context arrays). ext4_ext_insert_extent() handles
MAPPED -> PUBLISHED state transitions with the context on extent
insertion success.

The allocation context tracks only essential state:

• physical run (start + length)
• lifecycle state (4 values)
• accounting_applied (needed for ALLOCATED -> ROLLED_BACK)

Note 1: I used poolside/laguna-s-2.1:free AI to help the implementation of this algorithm,
It was instructed specifically NOT to take code from linux. Plus I had to review an fix it several times.
Note 2: I have a WIP implementation of softupdates on top of it.

Test Plan

I tested this with fsx : no failures.
I ran dbench with 8 clients and the results are marginally better, not too
impressive, but perhaps it had something to do that I did in a ZVOL,
which is the only thing based on a spinning disk here.
It is also likely this helps with fragmentation: ext3/4 has turned off reallocblk.

Oldalloc:
Throughput 3850.7 MB/sec 8 clients 8 procs max_latency=26.015 ms

Newalloc:
Throughput 3964.99 MB/sec 8 clients 8 procs max_latency=11.642 ms