block: rewrite and split __bio_copy_iov()

block: rewrite and split __bio_copy_iov()

Rewrite __bio_copy_iov using the copy_page_{from,to}_iter helpers, and
split it into two simpler functions.

This commit should contain only literal replacements, without
functional changes.

Cc: Kent Overstreet <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Dongsu Park <[email protected]>
[hch: removed the __bio_copy_iov wrapper]
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>