Having to specify the mapping address is a pain.  Give hugetlbfs files a
file_operations.get_unmapped_area().

The implementation is in hugetlbfs rather than in arch code because it's
probably common to several architectures.  If the architecture has special
needs it can define HAVE_ARCH_HUGETLB_UNMAPPED_AREA and go it alone.  Just
like HAVE_ARCH_UNMAPPED_AREA.