powerpc/pseries/cmm: Switch to balloon_page_alloc()

balloon_page_alloc() will use GFP_HIGHUSER_MOVABLE in case we have
CONFIG_BALLOON_COMPACTION. This is now possible, as balloon pages are
movable with CONFIG_BALLOON_COMPACTION. Without
CONFIG_BALLOON_COMPACTION, GFP_HIGHUSER is used.

Note that apart from that, balloon_page_alloc() uses the following
flags:
    __GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN
And current code used:
    GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC

GFP_HIGHUSER/GFP_HIGHUSER_MOVABLE include
    __GFP_RECLAIM | __GFP_IO | __GFP_FS | __GFP_HARDWALL | __GFP_HIGHMEM

GFP_NOIO is __GFP_RECLAIM.

With CONFIG_BALLOON_COMPACTION, we essentially add:
    __GFP_IO | __GFP_FS | __GFP_HARDWALL | __GFP_HIGHMEM | __GFP_MOVABLE

Without CONFIG_BALLOON_COMPACTION, we essentially add:
    __GFP_IO | __GFP_FS | __GFP_HARDWALL | __GFP_HIGHMEM

I assume this is fine, as this is what all other balloon compaction
users use. If it turns out to be a problem, we could add __GFP_MOVABLE
manually if we have CONFIG_BALLOON_COMPACTION.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191031142933.10779-10-david@redhat.com