mm: fix NULL NODE_DATA dereference for memoryless nodes on boot

Commit d49004c5 ("arch, mm: consolidate initialization of nodes, zones
and memory map") moved free_area_init() from setup_arch() to
mm_core_init_early(), which runs after setup_arch() returns.

This changed the ordering relative to init_cpu_to_node() on x86.  Before
the commit, free_area_init() ran during paging_init() (called from
setup_arch()) *before* init_cpu_to_node().  After the commit, it runs
*after* init_cpu_to_node().

On machines with memoryless NUMA nodes (e.g., node 0 has CPUs but no
memory), this causes a NULL pointer dereference:

 1. numa_register_nodes() skips memoryless nodes: no alloc_node_data()
    and no node_set_online() for them.
 2. init_cpu_to_node() sets memoryless nodes online (they have CPUs)
    but does not allocate NODE_DATA.
 3. free_area_init() checks "if (!node_online(nid))" to decide whether
    to call alloc_offline_node_data(). Since the memoryless node is now
    online, the allocation is skipped, leaving NODE_DATA(nid) == NULL.
 4. The immediate "pgdat = NODE_DATA(nid)" dereferences NULL.

The crash happens before console_init(), so no output is visible without
earlyprintk.  With earlyprintk enabled, the following panic is observed:

 BUG: unable to handle page fault for address: 000000000002a1e0
 Oops: Oops: 0000 [#1] SMP NOPTI
 RIP: 0010:free_area_init_node+0x3a/0x540
 Call Trace:
  <TASK>
  free_area_init+0x331/0x4e0
  start_kernel+0x69/0x4a0
  x86_64_start_reservations+0x24/0x30
  x86_64_start_kernel+0x125/0x130
  common_startup_64+0x13e/0x148
  </TASK>
 Kernel panic - not syncing: Attempted to kill the idle task!

Fix this by checking "if (!NODE_DATA(nid))" instead of "if
(!node_online(nid))".  This directly tests whether the per-node data
structure needs to be allocated, regardless of the node's online status. 
This change is also safe for non-x86 architectures as they all allocate
NODE_DATA for every node including memoryless ones, so the check simply
evaluates to false with no change in behavior.

Link: https://lkml.kernel.org/r/20260222115702.3659-1-ming.lei@redhat.com


Fixes: d49004c5 ("arch, mm: consolidate initialization of nodes, zones and memory map")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>