This is done in addition to MIGRATE_ISOLATE pageblock merge avoidance.
It prepares for the upcoming removal of the MAX_ORDER-1 alignment
requirement for CMA and alloc_contig_range().

MIGRATE_HIGHATOMIC should not merge with other migratetypes like
MIGRATE_ISOLATE and MIGRARTE_CMA[1], so this commit prevents that too.

Remove MIGRATE_CMA and MIGRATE_ISOLATE from fallbacks list, since they
are never used.

[1] https://lore.kernel.org/linux-mm/20211130100853.GP3366@techsingularity.net/

Link: https://lkml.kernel.org/r/20220124175957.1261961-1-zi.yan@sent.comSigned-off-by: NZi Yan <ziy@nvidia.com>
Acked-by: NMel Gorman <mgorman@techsingularity.net>
Acked-by: NDavid Hildenbrand <david@redhat.com>
Acked-by: NVlastimil Babka <vbabka@suse.cz>
Acked-by: NMike Rapoport <rppt@linux.ibm.com>
Reviewed-by: NOscar Salvador <osalvador@suse.de>
Cc: Mike Rapoport <rppt@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>