kmemleak scan crash due to invalid PFNs

[Qian Cai]

On an arm64 ThunderX2 server, the first kmemleak scan would crash with
CONFIG_DEBUG_VM_PGFLAGS=y due to page_to_nid() found a pfn that is not directly
mapped. Hence, the page->flags is not initialized.

Reverted 9f1eb38e0e113 (mm, kmemleak: little optimization while scanning) fixed
the problem.

[  102.195320] Unable to handle kernel NULL pointer dereference at virtual
address 0000000000000006
[  102.204113] Mem abort info:
[  102.206921]   ESR = 0x96000005
[  102.209997]   Exception class = DABT (current EL), IL = 32 bits
[  102.215926]   SET = 0, FnV = 0
[  102.218993]   EA = 0, S1PTW = 0
[  102.222150] Data abort info:
[  102.225047]   ISV = 0, ISS = 0x00000005
[  102.228887]   CM = 0, WnR = 0
[  102.231866] user pgtable: 64k pages, 48-bit VAs, pgdp = (____ptrval____)
[  102.238572] [0000000000000006] pgd=0000000000000000, pud=0000000000000000
[  102.245448] Internal error: Oops: 96000005 [#1] SMP
[  102.264062] CPU: 60 PID: 1408 Comm: kmemleak Not tainted 5.0.0-rc2+ #8
[  102.280403] pstate: 60400009 (nZCv daif +PAN -UAO)
[  102.280409] pc : page_mapping+0x24/0x144
[  102.280415] lr : __dump_page+0x34/0x3dc
[  102.292923] sp : ffff00003a5cfd10
[  102.296229] x29: ffff00003a5cfd10 x28: 000000000000802f
[  102.301533] x27: 0000000000000000 x26: 0000000000277d00
[  102.306835] x25: ffff000010791f56 x24: ffff7fe000000000
[  102.312138] x23: ffff000010772f8b x22: ffff00001125f670
[  102.317442] x21: ffff000011311000 x20: ffff000010772f8b
[  102.322747] x19: fffffffffffffffe x18: 0000000000000000
[  102.328049] x17: 0000000000000000 x16: 0000000000000000
[  102.333352] x15: 0000000000000000 x14: ffff802698b19600
[  102.338654] x13: ffff802698b1a200 x12: ffff802698b16f00
[  102.343956] x11: ffff802698b1a400 x10: 0000000000001400
[  102.349260] x9 : 0000000000000001 x8 : ffff00001121a000
[  102.354563] x7 : 0000000000000000 x6 : ffff0000102c53b8
[  102.359868] x5 : 0000000000000000 x4 : 0000000000000003
[  102.365173] x3 : 0000000000000100 x2 : 0000000000000000
[  102.370476] x1 : ffff000010772f8b x0 : ffffffffffffffff
[  102.375782] Process kmemleak (pid: 1408, stack limit = 0x(____ptrval____))
[  102.382648] Call trace:
[  102.385091]  page_mapping+0x24/0x144
[  102.388659]  __dump_page+0x34/0x3dc
[  102.392140]  dump_page+0x28/0x4c
[  102.395363]  kmemleak_scan+0x4ac/0x680
[  102.399106]  kmemleak_scan_thread+0xb4/0xdc
[  102.403285]  kthread+0x12c/0x13c
[  102.406509]  ret_from_fork+0x10/0x18
[  102.410080] Code: d503201f f9400660 36000040 d1000413 (f9400661)
[  102.416357] ---[ end trace 4d4bd7f573490c8e ]---
[  102.420966] Kernel panic - not syncing: Fatal exception
[  102.426293] SMP: stopping secondary CPUs
[  102.431830] Kernel Offset: disabled
[  102.435311] CPU features: 0x002,20000c38
[  102.439223] Memory Limit: none
[  102.442384] ---[ end Kernel panic - not syncing: Fatal exception ]---