powerpc/rtas: mandate RTAS syscall filtering

CONFIG_PPC_RTAS_FILTER has been optional but default-enabled since its
introduction. It's been enabled in enterprise distro kernels for a
while without causing ABI breakage that wasn't easily fixed, and it
prevents harmful abuses of the rtas syscall.

Let's make it unconditional.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221118150751.469393-10-nathanl@linux.ibm.com