Add a sched_yield() to work around low memory conditions in the current code.

Things seem to get stuck in low memory conditions where no bufs are available,
the reclamation path is called to wakeup the daemon, but no sleeping is done.
Because of this, we are stuck in a tight loop in the current process and
never run said reclamation path.

This was introduced in r289279 . This is only a temporary workaround
to restore system usefulness until the more permanent solutions can be
found.

Tested:

• Carambola2, 64MB (and 32MB by manual config.)