Fix locking order reversal in the TCP ratelimit code by moving
destructors outside the rsmtx mutex.

Witness message:
lock order reversal: (sleepable after non-sleepable)

1st tcp_rs_mtx (rsmtx) @ sys/netinet/tcp_ratelimit.c:242
2nd sysctl lock (sysctl lock) @ sys/kern/kern_sysctl.c:607

Backtrace:
witness_debugger
witness_checkorder
_rm_wlock_debug
sysctl_ctx_free
rs_destroy
epoch_call_task
gtaskqueue_run_locked
gtaskqueue_thread_loop

Discussed with: rrs@
Sponsored by: Mellanox Technologies