btrfs: Move btrfs_raid_array to public

This array is used to record attributes of each raid type,
make it public, and many functions will benifit with this array.

For example, num_tolerated_disk_barrier_failures(), we can
avoid complex conditions in this function, and get raid attribute
simply by accessing above array.

It can also make code logic simple, reduce duplication code, and
increase maintainability.

Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 2ca784a..75d6083 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -338,6 +338,10 @@
 	int ncopies;		/* how many copies to data has */
 };
 
+extern const struct btrfs_raid_attr btrfs_raid_array[BTRFS_NR_RAID_TYPES];
+
+extern const u64 btrfs_raid_group[BTRFS_NR_RAID_TYPES];
+
 struct map_lookup {
 	u64 type;
 	int io_align;