tap: do not close(fd) in net_init_tap_one

tap: do not close(fd) in net_init_tap_one

commit 5193e5fb (tap: factor out common tap initialization)
introduce net_init_tap_one(). But it's inappropriate that
we close fd in net_init_tap_one(), we should lay it in the
caller, becuase some callers needn't to close it if we get
the fd by monitor_handle_fd_param().

On the other hand, in other exceptional branches fd isn't
closed, so that's incomplete anyway.

Signed-off-by: Gonglei <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>