mm, hugetlb: integrate giga hugetlb more naturally to the allocation path

Gigantic hugetlb pages were ingrown to the hugetlb code as an alien
specie with a lot of special casing.  The allocation path is not an
exception.  Unnecessarily so to be honest.  It is true that the
underlying allocator is different but that is an implementation detail.

This patch unifies the hugetlb allocation path that a prepares fresh
pool pages.  alloc_fresh_gigantic_page basically copies
alloc_fresh_huge_page logic so we can move everything there.  This will
simplify set_max_huge_pages which doesn't have to care about what kind
of huge page we allocate.

Link: http://lkml.kernel.org/r/20180103093213.26329-3-mhocko@kernel.org


Signed-off-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Andrea Reale <ar@linux.vnet.ibm.com>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <zi.yan@cs.rutgers.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>