Make ifq and ifiq interface MP safe.

Make ifq and ifiq interface MP safe.

Rename ifq_set_maxlen() to ifq_init_maxlen().  This function neither
uses WRITE_ONCE() nor a mutex and is called before the ifq mutex
is initialized.  The new name expresses that it should be used only
during interface attach when there is no concurrency.

Protect ifq_len(), ifq_empty(), ifiq_len(), and ifiq_empty() with
READ_ONCE().  They can be used without lock as they only read a
single integer.

OK dlg@