to be cleared always in edmcadone(), otherwise if there is a write
via bounce buffer followed by read directly to buf, the read operation
would return trashed data (the buf data would get overwritten
by contents of bounce buffer in edmcadone()).
Reset b_resid as necessary when the i/o is done, too.
g/c some unneeded stuff, use lockmgr()-style locking in ed_[un]lock(),
better avoid some deadlocks
These changes make the driver quite a bit more stable. It's actually
reliable enough to be possible to newfs the drive and use it for
read/write filesystem now.
