don't rely upon subsequent bio_add_pc_page() calls failing

... they might actually succeed in some cases (when we are at the
queue-imposed segments limit, the next page is not mergable with
the last one we'd got in, but the first page covered by the next
iovec *is* mergable).  Make sure that once it's failed, we are
done with that bio.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>