mon: fix occasional message leak after session reset

Consider:

 - we get a message, put it on a wait list
 - the client session resets
 - we go back to process the message later and discard
   - _ms_dispatch returns false, but nobody drops the msg ref

Since we call _ms_dispatch() a lot internally, we need to always return
true when we are an internal caller.

Fixes: #9176
Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 19df386b2d36d716be2e6d02de0386fac9e7bc1f)