lazy-pages: use random read from page-pipe instead of splitting it

For the remote lazy pages case, to access pages in the middle of a pipe we
are splitting the page_pipe_buffers and iovecs and use splice() to move the
data between the underlying pipes. After the splits we get page_pipe_buffer
with single iovec that can be used to splice() the data further into the
socket.
This patch replaces the splitting and splicing with use of a helper pipe
and tee(). We tee() the pages from beginning of the pipe up to the last
requested page into a helper pipe, sink the unneeded head part into
/dev/null and we get the requested pages ready for splice() into the
socket.
This allows lazy-pages daemon to request the same page several time, which
is required to properly support fork() after the restore.
As added bonus we simplify the code and reduce amount of pipes that live in
the system.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>