[SCSI] Simplify error handling a bit

 - to_do was never set to SYM_EH_DO_COMPLETE, so remove that code
 - move the spinlocks inside the common error handler code path

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>