[BLOCK] bio: check for same page merge possibilities in __bio_add_page()

For filesystems with a blocksize < page size, we can merge same page
calls into the bio_vec at the end of the bio. This saves segments
on systems with a page size > the "normal" 4kb fs block size.

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