xfs: FSX_NONBLOCK is not used

It is set if the filp is set ot non-blocking, but the flag is not
used anywhere. Hence we can kill it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>