raid10: increment write counter after bio is split

BugLink: http://bugs.launchpad.net/bugs/1677600

commit 9b622e2b upstream.

md pending write counter must be incremented after bio is split,
otherwise it gets decremented too many times in end bio callback and
becomes negative.
Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Reviewed-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/md/raid10.c

@@ -1072,6 +1072,8 @@ static void __make_request(struct mddev *mddev, struct bio *bio)
 	int max_sectors;
 	int sectors;
 
+	md_write_start(mddev, bio);
+
 	/*
 	 * Register the new request and wait if the reconstruction
 	 * thread has put up a bar for new requests.
@@ -1455,8 +1457,6 @@ static void make_request(struct mddev *mddev, struct bio *bio)
 		return;
 	}
 
-	md_write_start(mddev, bio);
-
 	do {
 
 		/*