[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>

include/asm-mips/mmzone.h

@@ -14,9 +14,6 @@
 #define kvaddr_to_nid(kvaddr)	pa_to_nid(__pa(kvaddr))
 #define pfn_to_nid(pfn)		pa_to_nid((pfn) << PAGE_SHIFT)
 
-/* XXX: FIXME -- wli */
-#define kern_addr_valid(addr)	(0)
-
 #endif /* CONFIG_DISCONTIGMEM */
 
 #endif /* _ASM_MMZONE_H_ */

include/asm-mips/pgtable.h

@@ -379,9 +379,7 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
 	__update_cache(vma, address, pte);
 }
 
-#ifndef CONFIG_NEED_MULTIPLE_NODES
 #define kern_addr_valid(addr)	(1)
-#endif
 
 #ifdef CONFIG_64BIT_PHYS_ADDR
 extern int remap_pfn_range(struct vm_area_struct *vma, unsigned long from, unsigned long pfn, unsigned long size, pgprot_t prot);