rman: Embed the mutex in struct rman instead of using a separate allocation

[jhb]

Summary

This used a separate allocation when rman was first imported (back
when the lock was a pre-SMPng "simplelock" instead of a mutex).

Reported by: des

[des]

Thanks. I wonder if the performance impact is measurable...

[jhb]

Hopefully rman routines are not used in any critical paths (attach/detach handlers should be a relatively rare path compared to, say, an interrupt handler)