x86: fix virt_addr_valid() with CONFIG_DEBUG_VIRTUAL=y, v2

virt_addr_valid() calls __pa(), which calls __phys_addr(). With
CONFIG_DEBUG_VIRTUAL=y, __phys_addr() will kill the kernel if the
address *isn't* valid. That's clearly wrong for virt_addr_valid().

We also incorporate the debugging checks into virt_addr_valid().

Signed-off-by: Vegard Nossum <vegardno@ben.ifi.uio.no>
Acked-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>