ck_bytelock: Fix deadlock for unslotted reader workloads.

There is an off-by-one for slot ID sizeof(bytelock->readers) + 1.
This patch fixes the handling of this slot ID. Based off a patch
submitted by Albi Kavo <albi.kavo@gma....>.
regressions: Add coverage for ck_bytelock off-by-one.