The code sniplet to walk all bio_vecs and free their pages is opencoded in
way to many places, so factor it into a helper.  Also convert the slightly
more complex cases in bio_kern_endio and __bio_copy_iov where we break
the freeing from an existing loop into a separate one.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NMing Lei <tom.leiming@gmail.com>
Signed-off-by: NJens Axboe <axboe@fb.com>