mm: fix follow_pfnmap API lockdep assert

The lockdep asserts for the new follow_pfnmap() API "knows" that a
pfnmap always has a vma->vm_file, since that's the only way to create
such a mapping.

And that's actually true for all the normal cases.  But not for the mmap
failure case, where the incomplete mapping is torn down and we have
cleared vma->vm_file because the failure occured before the file was
linked to the vma.

So this codepath does actually need to check for vm_file being NULL.
Reported-by: Jann Horn <jannh@google.com>
Fixes: 6da8e963 ("mm: new follow_pfnmap API")
Cc: Peter Xu <peterx@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -6350,7 +6350,8 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
 static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma)
 {
 #ifdef CONFIG_LOCKDEP
-	struct address_space *mapping = vma->vm_file->f_mapping;
+	struct file *file = vma->vm_file;
+	struct address_space *mapping = file ? file->f_mapping : NULL;
 
 	if (mapping)
 		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||