geom(9): struct bio KBI fix

The struct bio was changed after cb85c2e2e995 on the branch. To fix
this, move BIO_ERROR flag to another value, and now BIO_ERROR_COMPAT
occupies 0x1 instead. Also, introduce b_error_compat field at the place
where the old bio_error was.

This allows non-CAM(9) disk drivers and software volume manager modules
compiled against 15.0-RELEASE kernel to work on 15-STABLE kernel again.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D54327

Approved by: re (cperciva)