Refactor CONFIG_SPE check on Linux/powerpc

Commit 5401472 adds a check to call enable_kernel_spe and
disable_kernel_spe only if CONFIG_SPE is defined. Refactor this check
in a way similar to what CONFIG_ALTIVEC and CONFIG_VSX are checked, in
order to remove redundant kfpu_begin() and kfpu_end() implementations.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: WHR <msl0000023508@gmail.com>
Closes #14623