buffer: Add _memcopy_count to track total count of memcopy by rebuild/rebuild_page_aligned/c_str.

Using thie filed, we know the payload of
rebuild/rebuild_page_aligned/c_str and tune performance accroding.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
Reviewed-by: Sage Weil <sage@redhat.com>
ObjectStore: Don't use largest_data_off to calc data_align.

If largest_data_off % CEPH_PAGE_SIZE != 0, the get_data_aligment return
a erro value. This make the FileJouranl::align_bl to memcopy much data.

Tested-by: Sage Weil <sage@redhat.com>
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
ObjectStore: Fix a error comment.

Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>