sched: fix sched_domain sysctl registration again

commit  029190c5 (cpuset
sched_load_balance flag) was not tested SCHED_DEBUG enabled as
committed as it dereferences NULL when used and it reordered
the sysctl registration to cause it to never show any domains
or their tunables.

Fixes:

1) restore arch_init_sched_domains ordering
	we can't walk the domains before we build them

	presently we register cpus with empty directories (no domain
	directories or files).

2) make unregister_sched_domain_sysctl do nothing when already unregistered
	detach_destroy_domains is now called one set of cpus at a time
	unregister_syctl dereferences NULL if called with a null.

	While the the function would always dereference null if called
	twice, in the previous code it was always called once and then
	was followed a register.  So only the hidden bug of the
	sysctl_root_table not being allocated followed by an attempt to
	free it would have shown the error.

3) always call unregister and register in partition_sched_domains
	The code is "smart" about unregistering only needed domains.
	Since we aren't guaranteed any calls to unregister, always 
	unregister.   Without calling register on the way out we
	will not have a table or any sysctl tree.

4) warn if register is called without unregistering
	The previous table memory is lost, leaving pointers to the
	later freed memory in sysctl and leaking the memory of the
	tables.

Before this patch on a 2-core 4-thread box compiled for SMT and NUMA,
the domains appear empty (there are actually 3 levels per cpu).  And as
soon as two domains a null pointer is dereferenced (unreliable in this
case is stack garbage):

bu19a:~# ls -R /proc/sys/kernel/sched_domain/
/proc/sys/kernel/sched_domain/:
cpu0  cpu1  cpu2  cpu3

/proc/sys/kernel/sched_domain/cpu0:

/proc/sys/kernel/sched_domain/cpu1:

/proc/sys/kernel/sched_domain/cpu2:

/proc/sys/kernel/sched_domain/cpu3:

bu19a:~# mkdir /dev/cpuset
bu19a:~# mount -tcpuset cpuset /dev/cpuset/
bu19a:~# cd /dev/cpuset/
bu19a:/dev/cpuset# echo 0 > sched_load_balance 
bu19a:/dev/cpuset# mkdir one
bu19a:/dev/cpuset# echo 1 > one/cpus               
bu19a:/dev/cpuset# echo 0 > one/sched_load_balance 
Unable to handle kernel paging request for data at address 0x00000018
Faulting instruction address: 0xc00000000006b608
NIP: c00000000006b608 LR: c00000000006b604 CTR: 0000000000000000
REGS: c000000018d973f0 TRAP: 0300   Not tainted  (2.6.23-bml)
MSR: 9000000000009032 <EE,ME,IR,DR>  CR: 28242442  XER: 00000000
DAR: 0000000000000018, DSISR: 0000000040000000
TASK = c00000001912e340[1987] 'bash' THREAD: c000000018d94000 CPU: 2
..
NIP [c00000000006b608] .unregister_sysctl_table+0x38/0x110
LR [c00000000006b604] .unregister_sysctl_table+0x34/0x110
Call Trace:
[c000000018d97670] [c000000007017270] 0xc000000007017270 (unreliable)
[c000000018d97720] [c000000000058710] .detach_destroy_domains+0x30/0xb0
[c000000018d977b0] [c00000000005cf1c] .partition_sched_domains+0x1bc/0x230
[c000000018d97870] [c00000000009fdc4] .rebuild_sched_domains+0xb4/0x4c0
[c000000018d97970] [c0000000000a02e8] .update_flag+0x118/0x170
[c000000018d97a80] [c0000000000a1768] .cpuset_common_file_write+0x568/0x820
[c000000018d97c00] [c00000000009d95c] .cgroup_file_write+0x7c/0x180
[c000000018d97cf0] [c0000000000e76b8] .vfs_write+0xe8/0x1b0
[c000000018d97d90] [c0000000000e810c] .sys_write+0x4c/0x90
[c000000018d97e30] [c00000000000852c] syscall_exit+0x0/0x40

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>