automount: ack automount requests even when already mounted (#5916)

      If a process accesses an autofs filesystem while systemd is in the
middle of starting the mount unit on top of it, it is possible for the
autofs_ptype_missing_direct request from the kernel to be received after
the mount unit has been fully started:

  systemd forks and execs mount             ...
            ...                     access autofs, blocks
  mount exits                               ...
  systemd receives SIGCHLD                  ...
            ...                     kernel sends request
  systemd receives request                  ...

systemd needs to respond to this request, otherwise the kernel will
continue to block access to the mount point.