devfs_d_revalidate_wait adds to a waitqueue but never removes from it
again so we there's one entry full of reused stack space added on
each call (I wonder how this ever worked).

The function has a few more bugs (it effectivly does a sleep_on instead
of checking for the actual even and can't deal with negative dentries
at all), but I just had breakfast and don't want to poke into devfs
internals deeper - I still hope Adam's smalldevfs will get merged
anyway..