[PATCH] update blk_execute_rq to take an at_head parameter

Original From: Mike Christie <michaelc@cs.wisc.edu>

Modified to split out block changes (this patch) and SCSI pieces.

Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>