dm: fix unconditional IO throttle caused by REQ_PREFLUSH

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SUSE CVE-2025-38063

In the Linux kernel, the following vulnerability has been resolved: dm: fix unconditional IO throttle caused by REQPREFLUSH When a bio with REQPREFLUSH is submitted to dm, sendemptyflush generates a flushbio with REQOPWRITE | REQPREFLUSH | REQSYNC, which causes the flushbio to be throttled by...