mm, hugetlb: fix simple resv_huge_pages underflow on UFFDIO_COPY

The userfaultfd hugetlb tests cause a resv_huge_pages underflow.  This
happens when hugetlb_mcopy_atomic_pte() is called with !is_continue on
an index for which we already have a page in the cache.  When this
happens, we allocate a second page, double consuming the reservation,
and then fail to insert the page into the cache and return -EEXIST.

To fix this, we first check if there is a page in the cache which
already consumed the reservation, and return -EEXIST immediately if so.

There is still a rare condition where we fail to copy the page contents
AND race with a call for hugetlb_no_page() for this index and again we
will underflow resv_huge_pages.  That is fixed in a more complicated
patch not targeted for -stable.

Test:

  Hacked the code locally such that resv_huge_pages underflows produce a
  warning, then:

  ./tools/testing/selftests/vm/userfaultfd hugetlb_shared 10
	2 /tmp/kokonut_test/huge/userfaultfd_test && echo test success
...