block: clean up __bio_add_pc_page a bit

Share the bi_size update by moving the done label up, and duplicate
the bv_len update in the two callers to get rid of the bvec_merge
label.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>