Get unused fd flags

• http://lxr.free-electrons.com/source/fs/file.c#L516

516 int get_unused_fd_flags(unsigned flags)
517 {
518         return __alloc_fd(current->files, 0, rlimit(RLIMIT_NOFILE), flags);  //current 전역변수, RLIMIT_NOFILE is max number of open file
519 }