target/loop: Enable VARLEN CDB support

Field shost->max_cmd_len is used to inform Linux / the SCSI midlayer of
the maximum CDB size an LLD is capable of handling. Set this field to
SCSI_MAX_VARLEN_CDB_SIZE for target, to enable support for
variable-sized CDBs (0x7E).

Also remove the definition of TL_SCSI_MAX_CMD_LEN since it is now
redundant.

Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Vangelis Koukis <vkoukis@arrikto.com>
Reviewed-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>