tar: fix -x --overwrite bug (no --dereference, ! O_NOFOLLOW)

This bug was discovered on Solaris 8.  On older hosts lacking
O_NOFOLLOW, tar -x --overwrite (without --dereference) follows
symbolic links, causing the "extract over symlinks" test to fail.

* src/extract.c (open_output_file): If O_NOFOLLOW is needed but
does not work, check for a symlink separately.

src/extract.c

@@ -864,6 +864,20 @@ open_output_file (char const *file_name, int typeflag, mode_t mode,
 	}
     }
 
+  /* If O_NOFOLLOW is needed but does not work, check for a symlink
+     separately.  There's a race condition, but that cannot be avoided
+     on hosts lacking O_NOFOLLOW.  */
+  if (! O_NOFOLLOW && overwriting_old_files && ! dereference_option)
+    {
+      struct stat st;
+      if (fstatat (chdir_fd, file_name, &st, AT_SYMLINK_NOFOLLOW) == 0
+	  && S_ISLNK (st.st_mode))
+	{
+	  errno = ELOOP;
+	  return -1;
+	}
+    }
+
   fd = openat (chdir_fd, file_name, openflag, mode);
   if (0 <= fd)
     {