Address space from ring buffer memory mapped into user space should be cleaned up

[mikeagun]

Describe the bug

Currently the memory mapping for ring buffers is never cleaned up (until the process exits). This is only a leak of address space in the user process, not memory, but we should return a handle the user-space process can use to release the mapping.

OS information

No response

Steps taken to reproduce bug

1. Repeatedly open and close a ring buffer

Expected behavior

After closing the ring buffer, the address space in the user process should be released.

Actual outcome

The address space stays reserved until the process exits.

Additional details

No response