iscsi: always query max WRITE SAME length

iscsi: always query max WRITE SAME length

Max WRITE SAME length is also used when the UNMAP bit is zero, so it
should be queried even if LBPWS=0.  Same for the optimal transfer
length.

However, the write_zeroes_alignment only matters for UNMAP=1 so we
still restrict it to LBPWS=1.

Reviewed-by: Peter Lieven <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>