xfs: don't pass ioflags around in the ioctl path

Instead check the file pointer for the invisble I/O flag directly, and
use the chance to drop redundant arguments from the xfs_ioc_space
prototype.

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