xen-blkfront: fix double completion of split requests on resume

When a block request is too large for a single ring entry and the
backend does not support indirect descriptors, blkfront splits it across
two ring requests. This only happens when the frontend runs on a
64K-page kernel (e.g. arm64): there, even a single-page request may not
fit in one ring slot and must be split. blkif_ring_get_request() is
called twice and both shadow slots (shadow[id] and shadow[extra_id])
point at the *same* struct request, linked through associated_id.

blkif_completion() collapses the pair on the normal completion path,
recycling the second slot and completing the request once. The
suspend/resume walk in blkfront_resume() does not: it visits every
shadow slot with ->request set and calls blk_mq_end_request() or
re-queues ->request. For an in-flight split request it therefore
processes the shared struct request twice on resume/migration -- a
double completion.

Skip the secondary slot of a split request in the resume walk so each
logical request is processed exactly once. The secondary slot is the
linked one (associated_id != NO_ASSOCIATED_ID) that carries no
scatter-gather list (num_sg == 0); the first slot always keeps the sg
list. The bug is only reachable on suspend/resume or live migration of
such a guest, so it has no local reproducer.

Fixes: 6cc568339047 ("xen/blkfront: Handle non-indirect grant with 64KB pages")
Assisted-by: 0sec:claude-opus-4-8
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://patch.msgid.link/20260709100853.7489-1-doruk@0sec.ai
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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