In addition to checking whether a page is reserved before allocating
it to highmem, verify that it is valid memory.

Otherwise the kernel Oopses as below:

  mem auto-init: stack:off, heap alloc:off, heap free:off
  Kernel attempted to read user page (7df58) - exploit attempt? (uid: 0)
  BUG: Unable to handle kernel data access on read at 0x0007df58
  Faulting instruction address: 0xc01c8348
  Oops: Kernel access of bad area, sig: 11 [#1]
  BE PAGE_SIZE=4K SMP NR_CPUS=2 P2020RDB-PC
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 6.0.0-rc2-0caacb197b677410bdac81bc34f05235+ #121
  NIP:  c01c8348 LR: c01cb2bc CTR: 0000000a
  REGS: c10d7e20 TRAP: 0300   Not tainted  (6.0.0-rc2-0caacb197b677410bdac81bc34f05235+)
  MSR:  00021000 <CE,ME>  CR: 48044224  XER: 00000000
  DEAR: 0007df58 ESR: 00000000
  GPR00: c01cb294 c10d7f10 c1045340 00000001 00000004 c112bcc0 00000015 eedf1000
  GPR08: 00000003 0007df58 00000000 f0000000 28044228 00000200 00000000 00000000
  GPR16: 00000000 00000000 00000000 0275cb7a c0000000 00000001 0000075f 00000000
  GPR24: c1031004 00000000 00000000 00000001 c10f0000 eedf1000 00080000 00080000
  NIP free_unref_page_prepare.part.93+0x48/0x60
  LR  free_unref_page+0x84/0x4b8
  Call Trace:
    0xeedf1000 (unreliable)
    free_unref_page+0x5c/0x4b8
    mem_init+0xd0/0x194
    start_kernel+0x4c0/0x6d0
    set_ivor+0x13c/0x178
Reported-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Fixes: b0e0d68b ("powerpc/32: Allow fragmented physical memory")
Tested-by: Pali Rohár <pali@kernel.org>
[mpe: Trim oops]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f08cca5c46d67399c53262eca48e015dcf1841f9.1663695394.git.christophe.leroy@csgroup.eu