mm: change enum migrate_mode with bitwise type

mm: change enum migrate_mode with bitwise type

Change migrate_mode type to bitwise type because next patch will add
MIGRATE_DISCARD and it could be ORed with other attributes so it would be
better to change it with bitwise type.

Suggested by Michal Nazarewicz.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>