tap: Avoid extra iterations while closing file fd

Avoid iterations for fd 0, 1 & 2 when we are closing file fds in child process.
Signed-off-by: NPankaj Gupta <pagupta@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

net/tap.c

@@ -367,11 +367,8 @@ static int launch_script(const char *setup_script, const char *ifname, int fd)
     if (pid == 0) {
         int open_max = sysconf(_SC_OPEN_MAX), i;
 
-        for (i = 0; i < open_max; i++) {
-            if (i != STDIN_FILENO &&
-                i != STDOUT_FILENO &&
-                i != STDERR_FILENO &&
-                i != fd) {
+        for (i = 3; i < open_max; i++) {
+            if (i != fd) {
                 close(i);
             }
         }
@@ -452,11 +449,8 @@ static int net_bridge_run_helper(const char *helper, const char *bridge)
         char br_buf[6+IFNAMSIZ] = {0};
         char helper_cmd[PATH_MAX + sizeof(fd_buf) + sizeof(br_buf) + 15];
 
-        for (i = 0; i < open_max; i++) {
-            if (i != STDIN_FILENO &&
-                i != STDOUT_FILENO &&
-                i != STDERR_FILENO &&
-                i != sv[1]) {
+        for (i = 3; i < open_max; i++) {
+            if (i != sv[1]) {
                 close(i);
             }
         }