LU-12495 obdclass: generate random u64 max correctly

LU-12495 obdclass: generate random u64 max correctly

Generate random u64 max number correctly, and make it an obdclass
function lu_prandom_u64_max().

Fixes: 7a707d4828 (libcfs: replace cfs_rand() with prandom_u32_max())

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I2b94a42b42539be319f358d7af2a82dc8b26117c
Reviewed-on: https://review.whamcloud.com/35394
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>