mm, hugetlb: remove hugetlb_zero and hugetlb_infinity

mm, hugetlb: remove hugetlb_zero and hugetlb_infinity

They are unnecessary: "zero" can be used in place of "hugetlb_zero" and
passing extra2 == NULL is equivalent to infinity.

Signed-off-by: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Reviewed-by: Naoya Horiguchi <[email protected]>
Reviewed-by: Luiz Capitulino <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>