scsi: read host_busy via scsi_host_busy()

scsi: read host_busy via scsi_host_busy()

No functional change.

Just introduce scsi_host_busy() and replace the direct read of
scsi_host->host_busy with this new API.

Cc: Omar Sandoval <[email protected]>,
Cc: "Martin K. Petersen" <[email protected]>,
Cc: James Bottomley <[email protected]>,
Cc: Christoph Hellwig <[email protected]>,
Cc: Don Brace <[email protected]>
Cc: Kashyap Desai <[email protected]>
Cc: Mike Snitzer <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Laurence Oberman <[email protected]>
Cc: Bart Van Assche <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>