core/semaphore: Introduce ensure_space_for_waiters()
core/semaphore: Introduce ensure_space_for_waiters()

Useful in situations when we want to ensure that future wait()s don't
fail on allocation failure.

avikivity/seastar,avikivity/seastar,syuu1228/seastar,kjniemi/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,dreamsxin/seastar,dreamsxin/seastar,cloudius-systems/seastar,kjniemi/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,kjniemi/seastar,avikivity/seastar