buffer: rework list::append_zero

buffer: rework list::append_zero

Instead of blindly adding new bufferptrs, try to memset the remaining
space in append_buffer and try appending that part to the list,
possibly reducing number of needed bufferptrs in blocksize aligned
bufferlists. If that won't be enough, have the additional bufferptrs
page aligned.
Additionally, don't create extra bufferptrs if there's no remaining
zeros to append.

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>