powerpc/kasan: Fix kasan_remap_early_shadow_ro()

At the moment kasan_remap_early_shadow_ro() does nothing, because
k_end is 0 and k_cur < 0 is always true.

Change the test to k_cur != k_end, as done in
kasan_init_shadow_page_tables()

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Fixes: cbd18991 ("powerpc/mm: Fix an Oops in kasan_mmu_init()")
Cc: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4e7b56865e01569058914c991143f5961b5d4719.1583507333.git.christophe.leroy@c-s.fr