[POWERPC] spufs: Call spu_acquire_saved() before calculating the SPU note sizes

It makes sense to stop the SPU processes as soon as possible.  Also if we
dont acquire_saved() I think there's a possibility that the value in
csa.priv2.spu_lslr_RW won't be accurate.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>