powerpc/booke: fix fast syscall entry on SMP

Use r10 instead of r9 to calculate CPU offset as r9 contains
the value from SRR1 which is used later.

Fixes: 1a4b739b ("powerpc/32: implement fast entry for syscalls on BOOKE")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>