Btrfs: increase BTRFS_MAX_MIRRORS by one for dev replace

This change of the define is effective in all modes, it
is required and used only in the case when a device replace
procedure is running. The reason is that during an active
device replace procedure, the target device of the copy
operation is a mirror for the filesystem data as well that
can be used to read data in order to repair read errors on
other disks.
Signed-off-by: NStefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/ctree.h

@@ -48,7 +48,7 @@ struct btrfs_ordered_sum;
 
 #define BTRFS_MAGIC "_BHRfS_M"
 
-#define BTRFS_MAX_MIRRORS 2
+#define BTRFS_MAX_MIRRORS 3
 
 #define BTRFS_MAX_LEVEL 8