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>
buffer: Add a test for bufferlist::rebuild_page_aligned

Make the last prt of bufflist which is page-aligned don't change anthing
after rebuild_page_aligned.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>