x86/mm: Don't exceed the valid physical address space

valid_phys_addr_range() is used to sanity check the physical address range
of an operation, e.g., access to /dev/mem. It uses __pa(high_memory)
internally.

If memory is populated at the end of the physical address space, then
__pa(high_memory) is outside of the physical address space because:

   high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;

For the comparison in valid_phys_addr_range() this is not an issue, but if
CONFIG_DEBUG_VIRTUAL is enabled, __pa() maps to __phys_addr(), which
verifies that the resulting physical address is within the valid physical
address space of the CPU. So in the case that memory is populated at the
end of the physical address space, this is not true and triggers a
VIRTUAL_BUG_ON().

Use __pa(high_memory - 1) to prevent the conversion from going beyond
the end of valid physical addresses.

Fixes: be62a320 ("x86/mm: Limit mmap() of /dev/mem to valid physical addresses")
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Craig Bergstrom <craigb@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sander Eikelenboom <linux@eikelenboom.it>
Cc: Sean Young <sean@mess.org>

Link: https://lkml.kernel.org/r/20190326001817.15413-2-rcampbell@nvidia.com