buffer: Judge bufferlist whether need  rebuild

The function of rebuild() are:
a: return a contiguous ptr
b: if len % CEPH_PAGE_SIZE == 0, the return ptr must page aligned.

Before doing, it should check whether bufferlist alreay ok.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
@liewegas .Ok, i see, the other patch have you any suggestion? Thanks!