TetherController - clear both ebpf tether offload maps on startup

This clears out any stale state from a potential previous run.

(also fix a bug... get...MapFd() return -errno on error)

Test: build, atest
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I4a452280a3754ef05964630809efb8cca2bbb833
diff --git a/server/TetherController.cpp b/server/TetherController.cpp
index 16f272c..747174f 100644
--- a/server/TetherController.cpp
+++ b/server/TetherController.cpp
@@ -206,12 +206,14 @@
 
     // Open BPF maps, ignoring errors because the device might not support BPF offload.
     int fd = getTetherIngressMapFd();
-    if (fd != -1) {
+    if (fd >= 0) {
         mBpfIngressMap.reset(fd);
+        mBpfIngressMap.clear();
     }
     fd = getTetherStatsMapFd();
-    if (fd != -1) {
+    if (fd >= 0) {
         mBpfStatsMap.reset(fd);
+        mBpfStatsMap.clear();
     }
 }