powerpc/mm: Fix comparing pointer to 0 warning

Fixes coccicheck warning:

./arch/powerpc/mm/pgtable_32.c:87:11-12: WARNING comparing pointer to 0

Avoid pointer type value compared to 0.

Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1604976961-20441-1-git-send-email-kaixuxia@tencent.com