md/raid5: complete discard bios while reshape is active

make_discard_request() returns without completing the bio when reshape
is in progress. Discard callers block in submit_bio_wait()
waiting for a completion that never arrives.  The caller hangs in
uninterruptible sleep, and this does not resolve when reshape finishes.

Complete the bio with BLK_STS_AGAIN so userspace can retry after reshape,
consistent with the existing policy of not processing discard during
reshape.

Tested on a loop-backed RAID5 array during mdadm --grow: without this
patch, blkdiscard hangs in bio_await() and remains in uninterruptible
sleep after md reports "reshape done"; with this patch it returns
-EAGAIN instead.

Signed-off-by: Genjian Zhang <zhanggenjian@kylinos.cn>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/20260711161326.962336-1-zhanggenjian@126.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>

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