mm/mlock: drop dead code in count_mm_mlocked_page_nr()

The check for mm being null has never been needed since the only caller
has always passed in current->mm. Remove the check from
count_mm_mlocked_page_nr().

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Liam R. Howlett <[e-mail]>
Suggested-by: Lukas Bulwahn <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/mlock.c

···
 	unsigned long end;
 	VMA_ITERATOR(vmi, mm, start);
 
-	if (mm == NULL)
-		mm = current->mm;
-
 	/* Don't overflow past ULONG_MAX */
 	if (unlikely(ULONG_MAX - len < start))
 		end = ULONG_MAX;
 	else
 		end = start + len;
+
 	for_each_vma_range(vmi, vma, end) {
 		if (vma->vm_flags & VM_LOCKED) {
 			if (start > vma->vm_start)