blkparse: read 'magic' first

Read the 'magic' portion of 'struct blk_io_trace' first when reading the
tracefile and only if all magic checks succeed, read the rest of the
trace.

This is a preparation of supporting multiple trace protocol versions.

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>