[SCSI] add support for variable length extended commands

Add support for variable-length, extended, and vendor specific
CDBs to scsi-ml. It is now possible for initiators and ULD's
to issue these types of commands. LLDs need not change much.
All they need is to raise the .max_cmd_len to the longest command
they support (see iscsi patch).

- clean-up some code paths that did not expect commands to be
  larger than 16, and change cmd_len members' type to short as
  char is not enough.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

include/scsi/scsi_host.h

@@ -573,13 +573,11 @@ struct Scsi_Host {
 	/*
 	 * The maximum length of SCSI commands that this host can accept.
 	 * Probably 12 for most host adapters, but could be 16 for others.
+	 * or 260 if the driver supports variable length cdbs.
 	 * For drivers that don't set this field, a value of 12 is
-	 * assumed.  I am leaving this as a number rather than a bit
-	 * because you never know what subsequent SCSI standards might do
-	 * (i.e. could there be a 20 byte or a 24-byte command a few years
-	 * down the road?).  
+	 * assumed.
 	 */
-	unsigned char max_cmd_len;
+	unsigned short max_cmd_len;
 
 	int this_id;
 	int can_queue;