kernel: mempool: fix race condition in rt_mp_alloc

When thread wake up from waiting for memory, there is a chance that
there is no memory available in high pressure. So use a loop to check
again. Otherwise, there will be a NULL reference.
workqueue: fix the windows eol
Merge pull request #316 from grissiom/fix-mempool

kernel: mempool: fix race condition in rt_mp_alloc