SimpleMessenger: allow RESETSESSION whenever we forget an endpoint

In the past (e229f8451d37913225c49481b2ce2896ca6788a2) we decided to disable
reset of lossless Pipes, because lossless peers resetting caused trouble and
they can't forget about each other. But they actually can: if mark_down()
is called.

I can't figure out how else we could forget about a remote endpoint, so I think
it's okay if we tell them we reset in order to clean up state. That's desirable
so that we don't get into strange situations with out-of-whack counters.

Fixes: #10080
Backport: giant, firefly, dumpling

Signed-off-by: Greg Farnum <gfarnum@redhat.com>