usercopy: Avoid HIGHMEM pfn warning

When running on a system with >512MB RAM with a 32-bit kernel built with:

	CONFIG_DEBUG_VIRTUAL=y
	CONFIG_HIGHMEM=y
	CONFIG_HARDENED_USERCOPY=y

all execve()s will fail due to argv copying into kmap()ed pages, and on
usercopy checking the calls ultimately of virt_to_page() will be looking
for "bad" kmap (highmem) pointers due to CONFIG_DEBUG_VIRTUAL=y:

 ------------[ cut here ]------------
 kernel BUG at ../arch/x86/mm/physaddr.c:83!
 invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
 CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.3.0-rc8 #6
 Hardware name: Dell Inc. Inspiron 1318/0C236D, BIOS A04 01/15/2009
 EIP: __phys_addr+0xaf/0x100
 ...
 Call Trace:
  __check_object_size+0xaf/0x3c0
  ? __might_sleep+0x80/0xa0
  copy_strings+0x1c2/0x370
  copy_strings_kernel+0x2b/0x40
  __do_execve_file+0x4ca/0x810
  ? kmem_cache_alloc+0x1c7/0x370
  do_execve+0x1b/0x20
  ...

The check is from arch/x86/mm/physaddr.c:

	VIRTUAL_BUG_ON((phys_addr >> PAGE_SHIFT) > max_low_pfn);

Due...