Linux PPC: Fix build failures on kernels built without CONFIG_SPE

We do a simple ifdef to avoid calling enable_kernel_spe()/
disable_kernel_spe() on PowerPC.

Reported-by: Rich Ercolani <Rincebrain@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Tested-by: Georgy Yakovlev <gyakovlev@gentoo.org>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14233
Closes #14244