ether82563: avoid deadlock due to icansleep() trying to acquire Rbpool.Lock

ether82563: avoid deadlock due to icansleep() trying to acquire Rbpool.Lock

icansleep() violates the lock ordering due to the following cases:

rbfree(): ilock(Rbpool.Lock) -> wakeup(): spli(), lock(Rbpool.Rendez)
sleep(): splhi(), lock(Rbpool.Rendez) -> icansleep(): ilock(Rbpool.Lock)

erik fixed this moving the wakeup() out of the ilock() in rbfree(),
but i think it is an error to try acquiering a ilock in sleeps wait
condition function in general.

so this is what we do:

in the icansleep() function, we check for the *real* event we care about;
that is, if theres a buffer available in the Rbpool. this is to handle
the case when rbfree() makes a buffer available *before* it sees us
setting p->starve = 1.

p->starve is now just used to gate rbfree() from calling wakeup() as
an optimization.

this might cause spurious wakeups but they are not a problem. missed
wakeups is the thing we have to prevent.