ptr_ring: __ptr_ring_zero_tail micro optimization

__ptr_ring_zero_tail currently does the - 1 operation twice:
- during initialization of head
- at each loop iteration

Let's just do it in one place, all we need to do
is adjust the loop condition. this is better:
- a slightly clearer logic with less duplication
- uses prefix -- we don't need to save the old value
- one less - 1 operation - for example, when ring is empty
  we now don't do - 1 at all, existing code does it once

Text size shrinks from 15081 to 15050 bytes.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/bcd630c7edc628e20d4f8e037341f26c90ab4365.1758976026.git.mst@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: mstsirkin

include/linux/ptr_ring.h

@@ -248,15 +248,15 @@ static inline bool ptr_ring_empty_bh(struct ptr_ring *r)
  */
 static inline void __ptr_ring_zero_tail(struct ptr_ring *r, int consumer_head)
 {
-	int head = consumer_head - 1;
+	int head = consumer_head;
 
 	/* Zero out entries in the reverse order: this way we touch the
 	 * cache line that producer might currently be reading the last;
 	 * producer won't make progress and touch other cache lines
 	 * besides the first one until we write out all entries.
 	 */
-	while (likely(head >= r->consumer_tail))
-		r->queue[head--] = NULL;
+	while (likely(head > r->consumer_tail))
+		r->queue[--head] = NULL;
 
 	r->consumer_tail = consumer_head;
 }