powerpc/kuap: Remove __kuap_assert_locked()

__kuap_assert_locked() is redundant with
__kuap_get_and_assert_locked().

Move the verification of CONFIG_PPC_KUAP_DEBUG in kuap_assert_locked()
and make it call __kuap_get_and_assert_locked() directly.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1a60198a25d2ba38a37f1b92bc7d096435df4224.1634627931.git.christophe.leroy@csgroup.eu