xfs: use FS_XFLAG definitions directly

Now that the ioctls have been hoisted up to the VFS level, use
the VFs definitions directly and remove the XFS specific definitions
completely. Userspace is going to have to handle the change of this
interface separately, so removing the definitions from xfs_fs.h is
not an issue here at all.

Signed-off-by: Dave Chinner <dchinner@redhat.com>