md/raid10: fix writes_pending and barrier reference leaks on discard failures

In the Linux kernel, the following vulnerability has been resolved: md/raid10: fix writes_pending and barrier reference leaks on discard failures raid10_make_request() acquires a writes_pending reference with md_write_start() before calling raid10_handle_discard(). Several failure paths in raid10_handle_discard() complete the bio and return without releasing the corresponding reference, causing md_write_end() to be skipped. Call md_write_end() before returning from these failure paths to keep writes_pending accounting balanced. Additionally, discard split allocation failures can occur after wait_barrier() succeeds. Those paths return without calling allow_barrier(), leaking the associated barrier reference. Release the barrier before returning from those paths.