cgroup/cpuset: fix null-ptr-deref in rebuild_sched_domains_cpuslocked

A null-pointer-dereference bug was reported by syzbot:

Oops: general protection fault, probably for address 0xdffffc0000000000:
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: 0010:bitmap_subset include/linux/bitmap.h:433 [inline]
RIP: 0010:cpumask_subset include/linux/cpumask.h:836 [inline]
RIP: 0010:rebuild_sched_domains_locked kernel/cgroup/cpuset.c:967
RSP: 0018:ffffc90003ecfbc0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000020
RDX: ffff888028de0000 RSI: ffffffff8200f003 RDI: ffffffff8df14f28
RBP: 0000000000000000 R08: 0000000000000cc0 R09: 00000000ffffffff
R10: ffffffff8e7d95b3 R11: 0000000000000001 R12: 0000000000000000
R13: 00000000000f4240 R14: dffffc0000000000 R15: 0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2f463fff CR3: 000000003704c000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 rebuild_sched_domains_cpuslocked kernel/cgroup/cpuset.c:983 [inline]
 rebuild_sched_domains+0x21/0x40 kernel/cgroup/cpuset.c:990
 sched_rt_handler+0xb5/0xe0 kernel/sched/rt.c:2911
 proc_sys_call_handler+0x47f/0x5a0 fs/proc/proc_sysctl.c:600
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1070 fs/read_write.c:688
 ksys_write+0x12a/0x250 fs/read_write.c:740
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x106/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The issue occurs when generate_sched_domains() returns ndoms = 1 and
doms = NULL due to a kmalloc failure. This leads to a null-pointer
dereference when accessing doms in rebuild_sched_domains_locked().

Fix this by adding a NULL check for doms before accessing it.

Fixes: 6ee43047e8ad ("cpuset: Remove unnecessary checks in rebuild_sched_domains_locked")
Reported-by: syzbot+460792609a79c085f79f@syzkaller.appspotmail.com
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>