ARM: 9359/1: flush: check if the folio is reserved for no-mapping addresses

Since commit a4d5613c ("arm: extend pfn_valid to take into account
freed memory map alignment") changes the semantics of pfn_valid() to check
presence of the memory map for a PFN. A valid page for an address which
is reserved but not mapped by the kernel[1], the system crashed during
some uio test with the following memory layout:

 node   0: [mem 0x00000000c0a00000-0x00000000cc8fffff]
 node   0: [mem 0x00000000d0000000-0x00000000da1fffff]
 the uio layout is：0xc0900000, 0x100000

the crash backtrace like:

  Unable to handle kernel paging request at virtual address bff00000
  [...]
  CPU: 1 PID: 465 Comm: startapp.bin Tainted: G           O      5.10.0 #1
  Hardware name: Generic DT based system
  PC is at b15_flush_kern_dcache_area+0x24/0x3c
  LR is at __sync_icache_dcache+0x6c/0x98
  [...]
   (b15_flush_kern_dcache_area) from (__sync_icache_dcache+0x6c/0x98)
   (__sync_icache_dcache) from (set_pte_at+0x28/0x54)
   (set_pte_at) from (remap_pfn_range+0x1a0/0x274)
   (remap_pfn_range) from (uio_mmap+0x184/0x1b8 [uio])
   (uio_mmap [uio]) from (__mmap_region+0x264/0x5f4)
   (__mmap_region) from (__do_mmap_mm+0x3ec/0x440)
   (__do_mmap_mm) from (do_mmap+0x50/0x58)
   (do_mmap) from (vm_mmap_pgoff+0xfc/0x188)
   (vm_mmap_pgoff) from (ksys_mmap_pgoff+0xac/0xc4)
   (ksys_mmap_pgoff) from (ret_fast_syscall+0x0/0x5c)
  Code: e0801001 e2423001 e1c00003 f57ff04f (ee070f3e)
  ---[ end trace 09cf0734c3805d52 ]---
  Kernel panic - not syncing: Fatal exception

So check if PG_reserved was set to solve this issue.

[1]: https://lore.kernel.org/lkml/Zbtdue57RO0QScJM@linux.ibm.com/

Fixes: a4d5613c

 ("arm: extend pfn_valid to take into account freed memory map alignment")
Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>