scsi: core: Drop using the host_lock to protect async_scan race condition

scsi: core: Drop using the host_lock to protect async_scan race condition

Previously, host_lock was used to prevent bit-set conflicts in async_scan,
but this approach introduced naked reads in some code paths.

Convert async_scan from a bitfield to a bool type to eliminate bit-level
conflicts entirely. Use __guarded_by(&scan_mutex) to indicate that the
async_scan variable is protected by scan_mutex.

Signed-off-by: Chaohai Chen <wdhh6@aliyun.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Link: https://patch.msgid.link/20260305025125.3649517-1-wdhh6@aliyun.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>