geom_disk: fix the post-read KMSAN check

[chs]

Summary

in g_disk_done() fix several problems with the post-read KMSAN check:

• only check the range of the specific bio at hand, not the entire original bio.
• only check if the read completed successfully.
• only check if the bio is not BIO_UNMAPPED.

Found with KMSAN.

Sponsored by: Netflix
Assisted-By: Claude Code - Opus 5 (1M context)

[imp]

Conditional on considering rewording the comments.

sys/geom/geom_disk.c

257

I think this can be reworded to be briefer.

"BIO_READ transactions only have valid data when bio_error == 0"

I'm happy to see the resid handled correctly below :)

[markj]

sys/geom/geom_disk.c

259

When do you see unmapped BIOs with KMSAN enabled? We should be disabling unmapped I/O when the sanitizers are configured.