In order for hugetlbfs to operate, prefaulting the vma at mmap()-time
while simultaneously instantiating and performing lookups on its
ratcache entries is needed as an isolated operation.  This is
implemented as part of a different function within hugetlbpage.c that
ties it to inode and key lookup and allocation.

The following patch simply moves the code already present into its own
function, calls it, and makes it available for hugetlbfs to use.