bcache: ignore read-ahead request failure on backing device

bcache: ignore read-ahead request failure on backing device

When md raid device (e.g. raid456) is used as backing device, read-ahead
requests on a degrading and recovering md raid device might be failured
immediately by md raid code, but indeed this md raid array can still be
read or write for normal I/O requests. Therefore such failed read-ahead
request are not real hardware failure. Further more, after degrading and
recovering accomplished, read-ahead requests will be handled by md raid
array again.

For such condition, I/O failures of read-ahead requests don't indicate
real health status (because normal I/O still be served), they should not
be counted into I/O error counter dc->io_errors.

Since there is no simple way to detect whether the backing divice is a
md raid device, this patch simply ignores I/O failures for read-ahead
bios on backing device, to avoid bogus backing device failure on a
degrading md raid array.

Suggested-and-tested-by: Thorsten Knabe <linux@thorsten-knabe.de>
Signed-off-by: Coly Li <colyli@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>