target: Fix WRITE_SAME usage with transport_get_size

For all flavours of WRITE_SAME, we only expect to handle a single block
of data-out buffer payload, regardless of the number of logical blocks
presented in the CDB.  This patch changes all flavours of WRITE_SAME in
transport_generic_cmd_sequencer() to pass '1' into transport_get_size()
instead of the extracted 'sectors' to properly handle the default usage
of sg_write_same without the --xferlen parameter.

Reported-by: Eric Seppanen <eric@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@risingtidesystems.com>