blktrace: untangle if/else sequence in __blk_add_trace

blktrace: untangle if/else sequence in __blk_add_trace

Untangle the if/else sequence setting the trace action in
__blk_add_trace() and turn it into a switch statement for better
extensibility.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>