md: remove REQ_NOWAIT support from raid1/10/456

REQ_NOWAIT support in md personalities that can block internally is
fundamentally incomplete. While reads can avoid some blocking paths,
write requests can still encounter cases where one mirror succeeds while
another returns -EAGAIN. At that point md cannot distinguish queue
pressure from a real device failure, so it can neither record a bad
block nor safely retry the write without REQ_NOWAIT, leaving mirrors
with divergent data.

Rather than continue advertising REQ_NOWAIT support for personalities
that cannot implement it correctly, remove it from raid1, raid10 and
raid456. Keep REQ_NOWAIT for linear and raid0, which only remap bios to
their underlying devices; stacked limits will still clear the feature if
any component device lacks REQ_NOWAIT support.

Fixes: bf2c411bb1cf ("md: raid456 add nowait support")
Fixes: c9aa889b035f ("md: raid10 add nowait support")
Fixes: 5aa705039c4f ("md: raid1 add nowait support")
Fixes: f51d46d0e7cb ("md: add support for REQ_NOWAIT")
Suggested-by: Yu Kuai <yukuai@fygo.io>
Signed-off-by: Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/20260628142737.1051059-1-abd.masalkhi@gmail.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>

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