loader: want mechanism to avoid RA with bcache

[tsoome]

Summary

While we have mechanisms in place to protect ourselves against the read
behind the disk end, there is still one corner case. As the GPT
partition table has backup table at the end of the disk, and we yet
do not know the size of the disk (if the wrong size is provided by the
firmware/bios), we need to limit the reads to avoid read ahead in such case.

Test Plan

Tested the build + boot.

[allanjude]

Approved By: allanjude