bcache read ahead may attempt to read past end of disk

The new bcache code does not know the size of the disk, and therefore may attempt to read past the end of the disk while trying to fill its read-ahead cache.

This is usually not an issue, it fails gracefully on all of my machines, but some BIOSes seem to retry the reads for up to 30 seconds each, resulting in a long stall during boot

Submitted by: Toomas Soome <tsoome@me.com>
Reviewed by: jhb, np
Differential Revision: https://reviews.freebsd.org/D6109