Teach on_exit_reset() to discard pending cleanups for dsm.

If a postmaster child invokes fork() and then calls on_exit_reset, that
should be sufficient to let it exit() without breaking anything, but
dynamic shared memory broke that by not updating on_exit_reset() to
discard callbacks registered with dynamic shared memory segments.

Per investigation of a complaint from Tom Lane.

src/backend/storage/ipc/ipc.c

@@ -400,4 +400,5 @@ on_exit_reset(void)
 	before_shmem_exit_index = 0;
 	on_shmem_exit_index = 0;
 	on_proc_exit_index = 0;
+	reset_on_dsm_detach();
 }