Instead of trying to biowait on a buf that has errored, do the

Instead of trying to biowait on a buf that has errored, do the
right thing, throw it away by marking it as B_INVAL and setting
the B_ERROR flag and b_error to the errno that was returned.

B_RAW doesn't matter there.

Issue report my mlarkin@, this diff tested and confirmed to
fix the issue by mlarkin also, thanks!

OK krw@, beck@, deraadt@