dpif-linux: Fix file descriptor leak.

get_major() opens /proc/devices to get the openvswitch major number
but never closes the FD.

lib/dpif-linux.c

@@ -677,6 +677,8 @@ get_major(const char *target)
         }
     }
 
+    fclose(file);
+
     VLOG_ERR("%s: %s major not found (is the module loaded?)", fn, target);
     return -ENODEV;
 }