crush: fix potential weight overflow

crush: fix potential weight overflow

E.g.:
./bin/ceph osd crush reweight osd.0 32768

ID WEIGHT      TYPE NAME                                            UP/DOWN REWEIGHT PRIMARY-AFFINITY
-4 32770.00000 root default~hdd
-3           -     host gitbuilder-ceph-rpm-centos7-amd64-basic~hdd
 0           -         osd.0                                             up  1.00000          1.00000
 1     1.00000         osd.1                                             up  1.00000          1.00000
 2     1.00000         osd.2                                             up  1.00000          1.00000
-1 32770.00000 root default
-2           -     host gitbuilder-ceph-rpm-centos7-amd64-basic
 0           -         osd.0                                             up  1.00000          1.00000
 1     1.00000         osd.1                                             up  1.00000          1.00000
 2     1.00000         osd.2                                             up  1.00000          1.00000

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>