MFC: Improve readahead in loader

style: Fix leading whitespace in bcache.c
loader: Refactor readahead adjustment in bcache
loader bcache: Track unconsumed readahead
loader bcache: Allow readahead up to 256 kB I/Os

Sponsored by: https://www.patreon.com/cperciva

(cherry picked from commit 5a01dea7e8c9640605e5731a9fc7f600c07ace61)
(cherry picked from commit b841148bbbdc967c871e8742a6f0b7b17b2d1d91)
(cherry picked from commit 04b9b7c507c52daf7b2999329a50825db098151f)
(cherry picked from commit 248682a589159619aa5f2019e415a423e849e327)