powerpc/mm: sanity_check_fault() should work for all, not only BOOK3S

The verification and message introduced by commit 374f3f59
("powerpc/mm/hash: Handle user access of kernel address gracefully")
applies to all platforms, it should not be limited to BOOK3S.

Make the BOOK3S version of sanity_check_fault() the one for all,
and bail out earlier if not BOOK3S.

Fixes: 374f3f59

 ("powerpc/mm/hash: Handle user access of kernel address gracefully")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/fe199d5af3578d3bf80035d203a94d742a7a28af.1607491748.git.christophe.leroy@csgroup.eu