powerpc/64s/hash: Fix 128TB-512TB virtual address boundary case allocation
When allocating VA space with a hint that crosses 128TB, the SLB
addr_limit variable is not expanded if addr is not > 128TB, but the
slice allocation looks at task_size, which is 512TB. This results in
slice_check_fit() incorrectly succeeding because the slice_count
truncates off bit 128 of the requested mask, so the comparison to the
available mask succeeds.
Fix this by using mm->context.addr_limit instead of mm->task_size for
testing allocation limits. This causes such allocations to fail.