Allow EBADF in AutoRemoteSyscalls::retrieve_fd_arch to continue.

Is there a reason not to continue here?
Seems to just show up with "--fatal-errors --check-cached-mmaps".

bin/rr --fatal-errors --check-cached-mmaps record bin/pid_ns_reap

Avoids error message:
  [FATAL src/AutoRemoteSyscalls.cc:659:retrieve_fd_arch() errno: EBADF]
   (task 693273 (rec:693273) at time 308)
   -> Assertion `errno == ENOSYS' failed to hold. Failed in pidfd_getfd errno=EBADF

Affected tests:
  pid_ns_reap

src/AutoRemoteSyscalls.cc

@@ -655,7 +655,7 @@ template <typename Arch> ScopedFd AutoRemoteSyscalls::retrieve_fd_arch(int fd) {
     if (ret.is_open()) {
       return ret;
     }
-    ASSERT(t, errno == ENOSYS) << "Failed in pidfd_getfd errno=" << errno_name(errno);
+    ASSERT(t, errno == ENOSYS || errno == EBADF) << "Failed in pidfd_getfd errno=" << errno_name(errno);
   }
 
   // Clear out any pending message in the socket.