userfaultfd: remove unnecessary WARN_ON() in __mcopy_atomic_hugetlb()

These warning here is to make sure address(dst_addr) and length(len -
copied) are huge page size aligned.

While this is ensured by:

    dst_start and len is huge page size aligned
    dst_addr equals to dst_start and increase huge page size each time
    copied increase huge page size each time

This means these warnings will never be triggered.

Link: http://lkml.kernel.org/r/20190927070032.2129-2-richardw.yang@linux.intel.com
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>