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 writes_pending leak on write request failures raid10_make_request() acquires a writes_pending reference with md_write_start() before dispatching write requests. Several failure paths in raid10_write_request() complete the bio and return without reaching the normal write completion path, causing the corresponding md_write_end() to be skipped. Make raid10_write_request() return a status indicating whether the write request was successfully queued. This allows raid10_make_request() to release the writes_pending reference with md_write_end() when a write request fails.