btrfs: join DEV_STATS ioctls to one

Commit c11d2c236cc260b36 (Btrfs: add ioctl to get and reset the device
stats) introduced two ioctls doing almost the same thing distinguished
by just the ioctl number which encodes "do reset after read". I have
suggested

http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg16604.html

to implement it via the ioctl args. This hasn't happen, and I think we
should use a more clean way to pass flags and should not waste ioctl
numbers.

CC: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index ecaad40..957bf39 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4890,8 +4890,7 @@
 }
 
 int btrfs_get_dev_stats(struct btrfs_root *root,
-			struct btrfs_ioctl_get_dev_stats *stats,
-			int reset_after_read)
+			struct btrfs_ioctl_get_dev_stats *stats)
 {
 	struct btrfs_device *dev;
 	struct btrfs_fs_devices *fs_devices = root->fs_info->fs_devices;
@@ -4909,7 +4908,7 @@
 		printk(KERN_WARNING
 		       "btrfs: get dev_stats failed, not yet valid\n");
 		return -ENODEV;
-	} else if (reset_after_read) {
+	} else if (stats->flags & BTRFS_DEV_STATS_RESET) {
 		for (i = 0; i < BTRFS_DEV_STAT_VALUES_MAX; i++) {
 			if (stats->nr_items > i)
 				stats->values[i] =