Increase MTX_POOL_SLEEP_SIZE from 128 to 1024.

This value remained unchanged for 15 years, and now this bump reduces
lock spinning in GEOM and BIO layers while doing ~1.6M IOPS to 4 NVMe
on 72-core system from ~25% to ~5% by the cost of additional 28KB RAM.

While there, align struct mtx_pool fields to cache lines.

MFC after: 1 month