bufferlist: Don't call rebuild(), c_strt() call it if bufferlist not contiguous.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
Merge pull request #2739 from majianpeng/fix2

buffer: Judge bufferlist whether need  rebuild

Reviewed-by: Haomai Wang <haomaiwang@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>