ck_bytelock: Fix downgrade path from writer to reader.

An off-by-one was introduced in downgrade path from writer.
This can cause deadlock if a writer downgrades from a write lock.

Pointed out by Jeffrey Birnbaum <jmb...@...>.

include/ck_bytelock.h

@@ -124,7 +124,7 @@ ck_bytelock_read_lock(struct ck_bytelock *bytelock, unsigned int slot)
 {
 
 	if (ck_pr_load_uint(&bytelock->owner) == slot) {
-		ck_pr_store_8(&bytelock->readers[slot], true);
+		ck_pr_store_8(&bytelock->readers[slot - 1], true);
 		ck_pr_fence_strict_store();
 		ck_pr_store_uint(&bytelock->owner, 0);
 		return;