ITER_PIPE: helpers for adding pipe buffers

There are only two kinds of pipe_buffer in the area used by ITER_PIPE.

1) anonymous - copy_to_iter() et.al. end up creating those and copying
data there.  They have zero ->offset, and their ->ops points to
default_pipe_page_ops.

2) zero-copy ones - those come from copy_page_to_iter(), and page
comes from caller.  ->offset is also caller-supplied - it might be
non-zero.  ->ops points to page_cache_pipe_buf_ops.

Move creation and insertion of those into helpers - push_anon(pipe, size)
and push_page(pipe, page, offset, size) resp., separating them from
the "could we avoid creating a new buffer by merging with the current
head?" logics.

Acked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>