um: Fix time-travel=inf-cpu with xor/raid6

Today, I erroneously built a time-travel configuration with btrfs
enabled, and noticed it cannot boot in time-travel=inf-cpu mode,
both xor and raid6 speed measurement gets stuck.

For xor, work around it by picking the first algorithm if inf-cpu
mode is enabled.

For raid6, I didn't find such a workaround, so disallow enabling
time-travel mode if RAID6_PQ_BENCHMARK is enabled.

With this, and RAID6_PQ_BENCHMARK disabled, I can boot a kernel
that has btrfs enabled in time-travel=inf-cpu mode.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>