When CONFIG_PPC_DEBUG_WX, note_prot_wx() is useless.

Get out of it early and inconditionnally in that case,
so that GCC can kick all the code out.
Signed-off-by: NChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ff6c8f631bd4ce3a10e0cc241eb569816187bc20.1565786091.git.christophe.leroy@c-s.fr