Btrfs: fix FS_IOC_SETFLAGS ioctl

Steps to reproduce the bug:

  - Call FS_IOC_SETLFAGS ioctl with flags=FS_COMPR_FL
  - Call FS_IOC_SETFLAGS ioctl with flags=0
  - Call FS_IOC_GETFLAGS ioctl, and you'll see FS_COMPR_FL is still set!

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