We support mixed merge for requests/bios with different fastfail
settings. When request fails, each time we only handle the portion
with same failfast setting, then bios with failfast can be failed
immediately, and bios without failfast can be retried.

The idea is pretty good, but the current implementation has several
defects:

1) initially RA bio doesn't set failfast, however bio merge code
doesn't consider this point, and just check its failfast setting for
deciding if mixed merge is required. Fix this issue by adding helper
of bio_failfast().

2) when merging bio to request front, if this request is mixed
merged, we have to sync request's faifast setting with 1st bio's
failfast. Fix it by calling blk_update_mixed_merge().

3) when merging bio to request back, if this request is mixed
merged, we have to mark the bio as failfast, because blk_update_request
simply updates request failfast with 1st bio's failfast. Fix
it by calling blk_update_mixed_merge().

Fixes one normal EXT4 READ IO failure issue, because it is observed
that the normal READ IO is merged with RA IO, and the mixed merged
request has different failfast setting with 1st bio's, so finally
the normal READ IO doesn't get retried.

Cc: Tejun Heo <tj@kernel.org>
Fixes: 80a761fd ("block: implement mixed merge of different failfast requests")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230209125527.667004-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>