bpf: fix numa_node validation

syzkaller reported crashes in bpf map creation or map update [1]

Problem is that nr_node_ids is a signed integer,
NUMA_NO_NODE is also an integer, so it is very tempting
to declare numa_node as a signed integer.

This means the typical test to validate a user provided value :

        if (numa_node != NUMA_NO_NODE &&
            (numa_node >= nr_node_ids ||
             !node_online(numa_node)))

must be written :

        if (numa_node != NUMA_NO_NODE &&
            ((unsigned int)numa_node >= nr_node_ids ||
             !node_online(numa_node)))

[1]
kernel BUG at mm/slab.c:3256!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 2946 Comm: syzkaller916108 Not tainted 4.13.0-rc7+ #35
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801d2bc60c0 task.stack: ffff8801c0c90000
RIP: 0010:____cache_alloc_node+0x1d4/0x1e0 mm/slab.c:3292
RSP: 0018:ffff8801c0c97638 EFLAGS: 00010096
RAX: ffffffffffff8b7b RBX: 0000000001080220 RCX: 0000000000000000
RDX: 00000000ffff8b7b RSI: 0000000001080220 RDI: ffff8801dac00040
RBP: ffff8801c0c976c0 R08: 0000000000000000 R09: 0000000000000000
R10: ffff8801c0c97620 R11: 0000000000000001 R12: ffff8801dac00040
R13: ffff8801dac00040 R14: 0000000000000000 R15: 00000000ffff8b7b
FS:  0000000002119940(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020001fec CR3: 00000001d2980000 CR4: 00000000001406f0
Call Trace:
 __do_kmalloc_node mm/slab.c:3688 [inline]
 __kmalloc_node+0x33/0x70 mm/slab.c:3696
 kmalloc_node include/linux/slab.h:535 [inline]
 alloc_htab_elem+0x2a8/0x480 kernel/bpf/hashtab.c:740
 htab_map_update_elem+0x740/0xb80 kernel/bpf/hashtab.c:820
 map_update_elem kernel/bpf/syscall.c:587 [inline]
 SYSC_bpf kernel/bpf/syscall.c:1468 [inline]
 SyS_bpf+0x20c5/0x4c40 kernel/bpf/syscall.c:1443
 entry_SYSCALL_64_fastpath+0x1f/0xbe
RIP: 0033:0x440409
RSP: 002b:00007ffd1f1792b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440409
RDX: 0000000000000020 RSI: 0000000020006000 RDI: 0000000000000002
RBP: 0000000000000086 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000401d70
R13: 0000000000401e00 R14: 0000000000000000 R15: 0000000000000000
Code: 83 c2 01 89 50 18 4c 03 70 08 e8 38 f4 ff ff 4d 85 f6 0f 85 3e ff ff ff 44 89 fe 4c 89 ef e8 94 fb ff ff 49 89 c6 e9 2b ff ff ff <0f> 0b 0f 0b 0f 0b 66 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41
RIP: ____cache_alloc_node+0x1d4/0x1e0 mm/slab.c:3292 RSP: ffff8801c0c97638
---[ end trace d745f355da2e33ce ]---
Kernel panic - not syncing: Fatal exception

Fixes: 96eabe7a ("bpf: Allow selecting numa node during map creation")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>