pollable_fd_state: Mark destructor protected and make non-virtual

The pollable_fd_state is base class for backend-specific fd states.
Those are created and destroyed via backend virtual calls. In that case,
base class destructor doesn't need to be virtual, deleting an object via
derived pointer would call correct destructor on its own. Also, the base
class destructor is marked protected to explicitly disallow deletion via
base-class.

Signed-off-by: Pavel Emelyanov <[email protected]>

include/seastar/core/internal/pollable_fd.hh

@@ -62,7 +62,6 @@ using pollable_fd_state_ptr = boost::intrusive_ptr<pollable_fd_state>;
 class pollable_fd_state {
     unsigned _refs = 0;
 public:
-    virtual ~pollable_fd_state() {}
     struct speculation {
         int events = 0;
         explicit speculation(int epoll_events_guessed = 0) : events(epoll_events_guessed) {}
@@ -122,6 +121,7 @@ public:
 protected:
     explicit pollable_fd_state(file_desc fd, speculation speculate = speculation())
         : fd(std::move(fd)), events_known(speculate.events) {}
+    ~pollable_fd_state() {};
 private:
     void maybe_no_more_recv();
     void maybe_no_more_send();