raw/ioat: fix ring space checks

raw/ioat: fix ring space checks

When enqueuing a descriptor, when checking that there is at least one
slot free for the current descriptor and a later batch descriptor, we
need to test for both two free and one free, in case the last write
was a batch descriptor which is allowed to use the "spare" slot.

Similarly, when computing the free space in the ring to return to the
user, we need to take account of the same condition, so that we do not
return a "-1" ring space value, by blindly subtracting "2".

Fixes: 245efe544d8e ("raw/ioat: report status of completed jobs")

Reported-by: Jiayu Hu <jiayu.hu@intel.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Kevin Laatz <kevin.laatz@intel.com>