scsi: use external buffer for command logging

Use an external buffer for __scsi_print_command() and move command
logging over to use the per-cpu logging buffer.  With that we can
guarantee the command always will always be formatted in one line.
So we can even print out a variable length command correctly across
several lines. Finally rename __scsi_print_command() to
__scsi_format_comment() to better reflect the functionality.

Tested-by: Robert Elliott <elliott@hp.com>
Reviewed-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>