target: Enforce hw_max_sectors for SCF_SCSI_DATA_SG_IO_CDB

Instead of depending upon a max_sectors value that may be set via
configfs based upon original HW queue limitations, go ahead and convert to using
the hw_max_sectors reported by the backend device in order to determine when
to reject an I/O's who's sector count exceeds what is supported by the backend
with a single se_cmd descriptor.

It addresses a potential case where se_dev_attrib.max_sectors for IBLOCK
backends has already been set via queue_max_sectors() to something small
like max_sectors=32 (LVM, DRBD may do this), resulting typically sized
SCF_SCSI_DATA_SG_IO_CDB to be incorrectly rejected with invalid_cdb_field
in transport_generic_cmd_sequencer().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Roland Dreier <roland@purestorage.com>
Cc: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>