buffer: In rebuild_page_aligned for the last ptr is page aligned, no need call rebuild().



This only happen for the last ptr. Because rebuild() don't change the len
of ptr, so if last ptr isn't page-size aligned but is page aligned, the
rebuild() don't change anything.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
aligned bufferptr mean is_page_aliged && p->is_n_page_sized?
bufferlist buffer;
 bufferptr ptr(buffer::create_page_aligned(2));
buffer.push_back(ptr)

If bufferlist only has a ptr. The function rebuild_page_aligned only make page aligned. But for the n_page_sized, it depend on the length. except we can pad data.