admin/server: ensure that a true expire-never logger is never expired

previously, this sequence of actions

set log-level raft trace 10
set log-level raft error 0

would result in raft logging at error level for only 10 seconds, despite
the legit request to expire never.

this is because in the code, the second request would not correctly
reset the expire field if a previous name-level_reset was already in the
map used for this operation

(cherry picked from commit d781fb1)

src/v/redpanda/admin/server.cc

@@ -1437,17 +1437,20 @@ void admin_server::register_config_routes() {
 
           ss::global_logger_registry().set_logger_level(name, new_level);
 
-          // expires=0 is same as not specifying it at all
-          if (expires_v / 1s > 0) {
-              auto when = ss::timer<>::clock::now() + expires_v;
-              auto res = _log_level_resets.try_emplace(name, cur_level, when);
-              if (!res.second) {
-                  res.first->second.expires = when;
+          auto when = [&]() -> std::optional<level_reset::time_point> {
+              // expires=0 is same as not specifying it at all
+              if (expires_v / 1s > 0) {
+                  return ss::timer<>::clock::now() + expires_v;
+              } else {
+                  // new log level never expires, but we still want an entry in
+                  // the resets map as a record of the default
+                  return std::nullopt;
               }
-          } else {
-              // new log level never expires, but we still want an entry in the
-              // resets map as a record of the default
-              _log_level_resets.try_emplace(name, cur_level, std::nullopt);
+          }();
+
+          auto res = _log_level_resets.try_emplace(name, cur_level, when);
+          if (!res.second) {
+              res.first->second.expires = when;
           }
 
           rsp.expiration = expires_v / 1s;