xfs: move xfs_fs.h to libxfs

Ioctl API definitions are shared with userspace, so move the header
file that defines them all to libxfs along with all the other code
shared with userspace.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>