mm,hugetlb: take hugetlb_lock before decrementing h->resv_huge_pages

In the Linux kernel, the following vulnerability has been resolved:

mm,hugetlb: take hugetlb_lock before decrementing h->resv_huge_pages

The h->*_huge_pages counters are protected by the hugetlb_lock, but alloc_huge_page has a corner case where it can decrement the counter outside of the lock.

This could lead to a corrupted value of h->resv_huge_pages, which we have observed on our systems.

Take the hugetlb_lock before decrementing h->resv_huge_pages to avoid a potential race.