dm-raid: Fix WARN_ON_ONCE check for sync_thread in raid_resume

In the Linux kernel, the following vulnerability has been resolved: dm-raid: Fix WARN_ON_ONCE check for sync_thread in raid_resume rm-raid devices will occasionally trigger the following warning when being resumed after a table load because DM_RECOVERY_RUNNING is set: WARNING: CPU: 7 PID: 5660 at drivers/md/dm-raid.c:4105 raid_resume+0xee/0x100 [dm_raid] The failing check is: WARN_ON_ONCE(test_bit(MD_RECOVERY_RUNNING, &mddev->recovery)); This check is designed to make sure that the sync thread isn't registered, but md_check_recovery can set MD_RECOVERY_RUNNING without the sync_thread ever getting registered. Instead of checking if MD_RECOVERY_RUNNING is set, check if sync_thread is non-NULL.