mm: remove bogus warning in copy_huge_pmd()

Sasha Levin reported the following warning being triggered

  WARNING: CPU: 28 PID: 35287 at mm/huge_memory.c:887 copy_huge_pmd+0x145/ 0x3a0()
  Call Trace:
    copy_huge_pmd+0x145/0x3a0
    copy_page_range+0x3f2/0x560
    dup_mmap+0x2c9/0x3d0
    dup_mm+0xad/0x150
    copy_process+0xa68/0x12e0
    do_fork+0x96/0x270
    SyS_clone+0x16/0x20
    stub_clone+0x69/0x90

This warning was introduced by "mm: numa: Avoid unnecessary disruption
of NUMA hinting during migration" for paranoia reasons but the warning
is bogus.  I was thinking of parallel races between NUMA hinting faults
and forks but this warning would also be triggered by a parallel reclaim
splitting a THP during a fork.  Remote the bogus warning.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Alex Thorlton <athorlton@sgi.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -883,9 +883,6 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		goto out_unlock;
 	}
 
-	/* mmap_sem prevents this happening but warn if that changes */
-	WARN_ON(pmd_trans_migrating(pmd));
-
 	if (unlikely(pmd_trans_splitting(pmd))) {
 		/* split huge page running from under us */
 		spin_unlock(src_ptl);