parisc: fix kernel BUG at arch/parisc/include/asm/mmzone.h:50

With CONFIG_DISCONTIGMEM=y and multiple physical memory areas,
cat /proc/kpageflags triggers this kernel bug:

kernel BUG at arch/parisc/include/asm/mmzone.h:50!
CPU: 2 PID: 7848 Comm: cat Tainted: G      D W 3.10.0-rc3-64bit #44
 IAOQ[0]: kpageflags_read0x128/0x238
 IAOQ[1]: kpageflags_read0x12c/0x238
 RP(r2): proc_reg_read0xbc/0x130
Backtrace:
 [<00000000402ca2d4>] proc_reg_read0xbc/0x130
 [<0000000040235bcc>] vfs_read0xc4/0x1d0
 [<0000000040235f0c>] SyS_read0x94/0xf0
 [<0000000040105fc0>] syscall_exit0x0/0x14

kpageflags_read() walks through the whole memory, even if some memory
areas are physically not available. So, we should better not BUG on an
unavailable pfn in pfn_to_nid() but just return the expected value -1 or
0.

Signed-off-by: Helge Deller <deller@gmx.de>