create_detached_idmapped_mount: avoid double close

fd_userns is defined as __do_close, so if it is not -EBADF, it will
be closed on exit.  So we need to either set it to -EBADF after closing
it, or just not manually close it.

My mistake in reviewing the previous patch.

Signed-off-by: Serge Hallyn <serge@hallyn.com>

shared/idmap/shift_linux.go

@@ -394,7 +394,6 @@ static int create_detached_idmapped_mount(const char *path, const char *fstype)
 	if (ret < 0)
 		return -errno;
 
-	close(fd_userns);
 	return 0;
 }
 */