btrfs: add GET_SUPPORTED_FEATURES to the control device ioctls

The control device is accessible when no filesystem is mounted and we
may want to query features supported by the module. This is already
possible using the sysfs files, this ioctl is for parity and
convenience.
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -4089,6 +4089,7 @@ void btrfs_test_inode_set_ops(struct inode *inode);
 
 /* ioctl.c */
 long btrfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
+int btrfs_ioctl_get_supported_features(struct file *file, void __user *arg);
 void btrfs_update_iflags(struct inode *inode);
 void btrfs_inherit_iflags(struct inode *inode, struct inode *dir);
 int btrfs_is_empty_uuid(u8 *uuid);

fs/btrfs/ioctl.c

@@ -5187,8 +5187,7 @@ static int btrfs_ioctl_set_fslabel(struct file *file, void __user *arg)
 	  .compat_ro_flags = BTRFS_FEATURE_COMPAT_RO_##suffix, \
 	  .incompat_flags = BTRFS_FEATURE_INCOMPAT_##suffix }
 
-static int btrfs_ioctl_get_supported_features(struct file *file,
-					      void __user *arg)
+int btrfs_ioctl_get_supported_features(struct file *file, void __user *arg)
 {
 	static const struct btrfs_ioctl_feature_flags features[3] = {
 		INIT_FEATURE_FLAGS(SUPP),

fs/btrfs/super.c

@@ -2163,6 +2163,10 @@ static long btrfs_control_ioctl(struct file *file, unsigned int cmd,
 			break;
 		ret = !(fs_devices->num_devices == fs_devices->total_devices);
 		break;
+	case BTRFS_IOC_GET_SUPPORTED_FEATURES:
+		ret = btrfs_ioctl_get_supported_features(NULL,
+				(void __user*)arg);
+		break;
 	}
 
 	kfree(vol);