NetBpfLoad: don't call bpfGetFdMapId in kernels < 4.14

The function is not available in older kernels.

But when we do call it, fail on error - should not happen.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I2695bf01762abcbc087a3bd67f1718e0e326576a
diff --git a/bpf/loader/NetBpfLoad.cpp b/bpf/loader/NetBpfLoad.cpp
index 5554104..c67be66 100644
--- a/bpf/loader/NetBpfLoad.cpp
+++ b/bpf/loader/NetBpfLoad.cpp
@@ -859,11 +859,13 @@
             }
         }
 
-        int mapId = bpfGetFdMapId(fd);
-        if (mapId == -1) {
-            if (isAtLeastKernelVersion(4, 14, 0))
-                ALOGE("bpfGetFdMapId failed, ret: %d [%d]", mapId, errno);
-        } else {
+        if (isAtLeastKernelVersion(4, 14, 0)) {
+            int mapId = bpfGetFdMapId(fd);
+            if (mapId == -1) {
+                const int err = errno;
+                ALOGE("bpfGetFdMapId failed, errno: %d", err);
+                return -err;
+            }
             ALOGI("map %s id %d", mapPinLoc.c_str(), mapId);
         }