[SCSI] fix sg leak when scsi_execute_async fails

Doug found a bug where if scsi_execute_async fails, we are leaking
sg resources. scsi_do_req never failed so we did not have to handle
that case before.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>