LU-11838 osp: atomic64_read() returns s64

LU-11838 osp: atomic64_read() returns s64

Since kernel 4.17 atomic64_read on x86_64 returns s64
instead of long.

Use %llu as the format specifier and explicitly cast it
to unsigned long long.

This patch is back-ported from the following one:
Lustre-commit: dc46952ecd1aa09e738b2de6b1a3076ecbaa740e
Lustre-change: https://review.whamcloud.com/34676

Test-Parameters:trivial
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I805d43251f24417e6405f5d087927c15cf531619
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35174
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>