LU-14016 libcfs: use atomic64_t for libcfs_kmem

LU-14016 libcfs: use atomic64_t for libcfs_kmem

libcfs_kmem keeps track of LNet's memory usage. It uses an
int type, so it could wrap around if usage grows beyond 2.14 GB.
Use atomic64_t to avoid this issue.

Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: If96fb8391c6ffb1924e47cef3dfca02eabc5f912
Reviewed-on: https://review.whamcloud.com/40168
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>