Use wrapping add in submission index

After about 4.2 billion submission this will overflow, make sure it
wraps around instead.

Note that due to the masking done on the tail below this will keep
working after wrapping around back to zero.

src/lib.rs

@@ -613,7 +613,7 @@ impl SubmissionQueue {
             .fetch_update(Ordering::AcqRel, Ordering::Acquire, |tail| {
                 if tail - kernel_read < shared.len {
                     // Still an entry available.
-                    Some(tail + 1) // TODO: handle overflows.
+                    Some(tail.wrapping_add(1))
                 } else {
                     None
                 }