block: cleanup and fix batch completion adding conditions

The conditions for whether or not a request is allowed adding to a
completion batch are a bit hard to read, and they also have a few
issues. One is that ioerror may indeed be a random value on passthrough,
and it's being checked unconditionally of whether or not the given
request is a passthrough request or not.

Rewrite the conditions to be separate for easier reading, and only check
ioerror for non-passthrough requests. This fixes an issue with bio
unmapping on passthrough, where it fails getting added to a batch. This
both leads to suboptimal performance, and may trigger a potential
schedule-under-atomic condition for polled passthrough IO.

Fixes: f794f335 ("block: add support for blk_mq_end_request_batch()")
Link: https://lore.kernel.org/r/20575f0a-656e-4bb3-9d82-dec6c7e3a35c@kernel.dk


Signed-off-by: Jens Axboe <axboe@kernel.dk>