Change crush_ln to provide 32 more digits.

Change crush_ln to provide 32 more digits.

32 more digits(more precision) were given by crush_ln, so we dont
 need to do left shift in bucket_straw2_choose now.

This change will slightly improve the quality of distribution.

expect                  169.664
osd     weight  count   adjusted
0       1       179     179
1       1.75    291     166
2       3.062   507     165
3       5.359   937     174
4       9.379   1581    168
5       16.41   2771    168
6       28.72   4847    168
7       50.27   8690    172
8       87.96   14924   169
9       153.9   26069   169
10      269.4   45706   169
11      471.4   79754   169
12      825     139847  169
13      1444    245707  170
14      2527    428190  169
std dev 12.4895
     vs 12.6005 (expected)

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>