[POWERPC] spufs: make mailbox functions handle multiple elements

Since libspe2 will provide a function that can read/write
multiple mailbox elements at once, the kernel should handle
that efficiently.

read/write on the three mailbox files can now access the
spe context multiple times to operate on any number of
mailbox data elements.

If the spu application keeps writing to its outbound
mailbox, the read call will pick up all the data in a
single system call.

Unfortunately, if the user passes an invalid pointer,
we may lose a mailbox element on read, since we can't
put it back. This probably impossible to solve, if the
user also accesses the mailbox through direct register
access.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>