From: Benjamin Herrenschmidt <benh@kernel.crashing.org>

This patch seem to have been lost, so here it is again.  It fixes an Ooops
on unregistering hwifs due to the device model now having mandatory
release() functions.  It also close the possible race we had on release if
the entry was in use (by or /sys typically) by using a semaphore waiting
for the release() to be called after doing an unregister.