xfs_io: fix random pread/pwrite to honor offset

xfs_io: fix random pread/pwrite to honor offset

xfs_io's pread & pwrite claim to support a random IO mode
where it will do random IOs between offset & offset+len.

However, offset was ignored, and we did the IOs between 0
and len instead.

Clang caught this by pointing out that the calculated/normalized
"offset" variable was never read.

(NB: If the range is larger than RAND_MAX, these functions don't
work, but that's always been true, so I'll leave it for another
day...)

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