mm: don't do validate_mm() unnecessarily and without mmap locking

This is an addition to commit ae80b40 ("mm: validate the mm before
dropping the mmap lock"), because it turns out there were two problems,
but lockdep just stopped complaining after finding the first one.

The do_vmi_align_munmap() function now drops the mmap lock after doing
the validate_mm() call, but it turns out that one of the callers then
immediately calls validate_mm() again.

That's both a bit silly, and now (again) happens without the mmap lock
held.

So just remove that validate_mm() call from the caller, but make sure to
not lose any coverage by doing that mm sanity checking in the error path
of do_vmi_align_munmap() too.

Reported-and-tested-by: kernel test robot <oliver.sang@intel.com>
Link: https://lore.kernel.org/lkml/ZKN6CdkKyxBShPHi@xsang-OptiPlex-9020/
Fixes: 408579c ("mm: Update do_vmi_align_munmap() return semantics")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: torvalds

mm/mmap.c

@@ -2571,6 +2571,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	__mt_destroy(&mt_detach);
 start_split_failed:
 map_count_exceeded:
+	validate_mm(mm);
 	return error;
 }
 
@@ -3019,12 +3020,9 @@ int do_vma_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 		bool unlock)
 {
 	struct mm_struct *mm = vma->vm_mm;
-	int ret;
 
 	arch_unmap(mm, start, end);
-	ret = do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
-	validate_mm(mm);
-	return ret;
+	return do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
 }
 
 /*