scsi: core: Remove an atomic instruction from the hot path

From scsi_init_command(), a function called by scsi_mq_prep_fn():

	/* zero out the cmd, except for the embedded scsi_request */
	memset((char *)cmd + sizeof(cmd->req), 0,
		sizeof(*cmd) - sizeof(cmd->req) + dev->host->hostt->cmd_size);

In other words, scsi_mq_prep_fn() clears scsi_cmnd.flags. Hence move the
clear_bit() call into the else branch, the only branch in which this code
is necessary.

See also commit f1342709 ("scsi: Do not rely on blk-mq for double
completions").

Cc: Keith Busch <keith.busch@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>