hugetlb: fix race in alloc_fresh_huge_page()

That static `nid' index needs locking.  Without it we can end up calling
alloc_pages_node() with an illegal node ID and the kernel crashes.

Acked-by: Gurudas Pai <gurudas.pai@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>