[MIPS] Drop 0 definition for kern_addr_valid

kern_addr_valid is currently only being used in kmem_ptr_validate which
is making some vague attempt at verfying the validity of an address.
Only IA-64, PARISC and x86-64 actually make some actual effort to verify
the validity of the pointer.  Most architecture definitions of
kern_addr_valid() just define it as 1; the Alpha and CONFIG_DISCONTIGMEM
on i386 and MIPS even as 0; the 0-definition will result in
kmem_ptr_validate always failing which in turn will cause d_validate to
always fail.  d_validate's only two users are smbfs and ncpfs, so the
0 definition ended breaking those ...

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>