Btrfs: Fix BTRFS_IOC_SUBVOL_SETFLAGS ioctl

- Check user-specified flags correctly
- Check the inode owership
- Search root item in root tree but not fs tree

Reported-by: Dan Rosenberg <drosenberg@vsecurity.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>