Allow "--write-behind=" to be done in grow mode.

From: Ian Dall <ian@beware.dropbear.id.au>

I have a small patch to mdadm which allows the write-behind amount to be
set a array grow time (instead of currently only at grow or create
time). I have tested this fairly extensively on some arrays built out of
loop back devices, and once on a real live array.

ChangeLog

@@ -1,3 +1,6 @@
+Changes Prior to this release
+    -   allow --write-behind to be set for --grow.
+
 Changes Prior to 2.6.2 release
     -   --fail detached and --remove faulty can be used to fail and
 	remove devices that are no longer physically present.

mdadm.c

@@ -844,6 +844,7 @@ int main(int argc, char *argv[])
 			bitmap_chunk = bitmap_chunk ? bitmap_chunk * 1024 : 512;
 			continue;
 
+		case O(GROW, WriteBehind):
 		case O(BUILD, WriteBehind):
 		case O(CREATE, WriteBehind): /* write-behind mode */
 			write_behind = DEFAULT_MAX_WRITE_BEHIND;