powerpc/32s: Fix build failure with CONFIG_PPC_KUAP_DEBUG

gpr2 is not a parametre of kuap_check(), it doesn't exist.

Use gpr instead.

Fixes: a68c31fc

 ("powerpc/32s: Implement Kernel Userspace Access Protection")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/ea599546f2a7771bde551393889e44e6b2632332.1587368807.git.christophe.leroy@c-s.fr