dpif: Don't rely on caller to keep dpif arg valid, in dpifmon_create().

dpif: Don't rely on caller to keep dpif arg valid, in dpifmon_create().

dpifmon_create() stored away the dpif pointer that it was passed and
continued to use it, but ofproto_create() didn't keep that dpif in a
constant place in memory, so dpifmon_poll() would randomly fail with
errors like "Bad file descriptor".  Having the dpifmon keep its own
fd to a dpif is more reliable.

Additional possible fix to Dan's problem.