LU-709 build: clean up libcfs/autoconf/lustre-libcfs.m4

1. cpu_online is found from v2.6.18 to latest
2. gfp_t is always defined from v2.6.18 to latest
3. HAVE_SCHED_SHOW_TASK is not used
4. task_struct has rcu field since v2.6.16
5. panic_notifier_list is atomic since v2.6.18
6. HAVE_NLMSG_MULTICAST_5ARGS is not used
7. HAVE_NETLINK and HAVE_NETLINK_NL2 are not used
8. kernel has kmem_cache from v2.6.18 to latest
9. HAVE_NETLINK_NS and HAVE_NETLINK_CBMUTEX is not used

Signed-off-by: Peng Tao <peng_tao@emc.com>
Change-Id: Ib10d4b9f19f96aa85f9be4d6f2dbaa7fb4cbbc34
Reviewed-on: http://review.whamcloud.com/2328
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>