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: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 46bd7d5..91ff078 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -48,7 +48,7 @@
 
 #define BTRFS_MAGIC "_BHRfS_M"
 
-#define BTRFS_MAX_MIRRORS 2
+#define BTRFS_MAX_MIRRORS 3
 
 #define BTRFS_MAX_LEVEL 8