A few (but not all) dm targets use a special EWOULDBLOCK error code for
failing REQ_RAHEAD requests that fail due to a lack of available resources.
But no one else knows about this magic code, and lower level drivers also
don't generate it when failing read-ahead requests for similar reasons.

So remove this special casing and ignore all additional error handling for
REQ_RAHEAD - if this was a real underlying error we'd get a normal read
once the real read comes in.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NBart Van Assche <Bart.VanAssche@sandisk.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NJens Axboe <axboe@fb.com>