nexthop: Fix splat with CONFIG_DEBUG_PREEMPT=y

Locally generated packets can increment the new nexthop statistics from
process context, resulting in the following splat [1] due to preemption
being enabled. Fix by using get_cpu_ptr() / put_cpu_ptr() which will
which take care of disabling / enabling preemption.

BUG: using smp_processor_id() in preemptible [00000000] code: ping/949
caller is nexthop_select_path+0xcf8/0x1e30
CPU: 12 PID: 949 Comm: ping Not tainted 6.8.0-rc7-custom-gcb450f605fae #11
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-1.fc38 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xbd/0xe0
 check_preemption_disabled+0xce/0xe0
 nexthop_select_path+0xcf8/0x1e30
 fib_select_multipath+0x865/0x18b0
 fib_select_path+0x311/0x1160
 ip_route_output_key_hash_rcu+0xe54/0x2720
 ip_route_output_key_hash+0x193/0x380
 ip_route_output_flow+0x25/0x130
 raw_sendmsg+0xbab/0x34a0
 inet_sendmsg+0xa2/0xe0
 __sys_sendto+0x2ad/0x430
 __x64_sys_sendto+0xe5/0x1c0
 do_syscall_64+0xc5/0x1d0
 entry_SYSCALL_64_after_hwframe+0x63/0x6b
[...]

Fixes: f4676ea ("net: nexthop: Add nexthop group entry stats")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240311162307.545385-5-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/nexthop.c

@@ -674,10 +674,11 @@ static void nh_grp_entry_stats_inc(struct nh_grp_entry *nhge)
 {
 	struct nh_grp_entry_stats *cpu_stats;
 
-	cpu_stats = this_cpu_ptr(nhge->stats);
+	cpu_stats = get_cpu_ptr(nhge->stats);
 	u64_stats_update_begin(&cpu_stats->syncp);
 	u64_stats_inc(&cpu_stats->packets);
 	u64_stats_update_end(&cpu_stats->syncp);
+	put_cpu_ptr(cpu_stats);
 }
 
 static void nh_grp_entry_stats_read(struct nh_grp_entry *nhge,