mm: allow pfnmap ->fault()s

Take out an assertion to allow ->fault handlers to service PFNMAP regions.
This is required to reimplement .nopfn handlers with .fault handlers and
subsequently remove nopfn.
Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Jes Sorensen <jes@sgi.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -2295,8 +2295,6 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	vmf.flags = flags;
 	vmf.page = NULL;
 
-	BUG_ON(vma->vm_flags & VM_PFNMAP);
-
 	ret = vma->vm_ops->fault(vma, &vmf);
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
 		return ret;