btrfs: use btrfs_raid_array for btrfs_get_num_tolerated_disk_barrier_failures()

btrfs_raid_array[] is used to define all raid attributes, use it
to get tolerated_failures in btrfs_get_num_tolerated_disk_barrier_failures(),
instead of complex condition in function.

It can make code simple and auto-support other possible raid-type in
future.

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 75d6083..04ff0ef 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -334,6 +334,7 @@
 	int dev_stripes;	/* stripes per dev */
 	int devs_max;		/* max devs to use */
 	int devs_min;		/* min devs needed */
+	int tolerated_failures; /* max tolerated fail devs */
 	int devs_increment;	/* ndevs has to be a multiple of this */
 	int ncopies;		/* how many copies to data has */
 };