log: Do not crash if logger is invoked from non-reactor thread
log: Do not crash if logger is invoked from non-reactor thread

Useful for adding temporary tracing to worker threads.

Message-Id: <c54eefb189398fc7ac0feb9cf69a8bd2430f21c4@scylladb.com>

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