um: vector: always reset vp->opened

um: vector: always reset vp->opened

If open fails, we have already set vp->opened, but it's
not reset so that any further attempts will just return
-ENXIO. Reset vp->opened even if close has nothing to do.

This helps e.g. with slirp4netns handling only a single
connection, you can then restart it and open the device
again.

Link: https://patch.msgid.link/20240703184622.df40c5c38461.Id4e20b48938c6019d99e6133227a34ac059db466@changeid
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>