msnd_pinnacle/msnd_classic do copy_{to,from}_user under a spinlock.

Taken out of spinlock (into a temp. buffer). Calls of msnd_fifo_{read,write}
always go from kernel buffer now, so we can drop the 'int user' argument in
them _and_ simplify error handling - all errors were from copy_..._user() and
now these are called directly by dsp_read()/dsp_write().