CVE-2026-72439: md/raid10: fix writes_pending leak on write request failures

In the Linux kernel, the following vulnerability has been resolved:

md/raid10: fix writespending leak on write request failures

raid10makerequest() acquires a writespending reference with mdwritestart() before dispatching write requests. Several failure paths in raid10writerequest() complete the bio and return without reaching the normal write completion path, causing the corresponding mdwriteend() to be skipped.

Make raid10writerequest() return a status indicating whether the write request was successfully queued. This allows raid10makerequest() to release the writespending reference with mdwriteend() when a write request fails.