target: Fix incorrect starting offset after MODE_SENSE refactoring

target: Fix incorrect starting offset after MODE_SENSE refactoring

This patch fixes a new off-by-one bug in the hardcoded starting offset of
spc_emulate_modesense() code that causes BLOCK DESCRIPTOR to be incorrectly
written within the MEDIUM TYPE buffer area of the mode parameter header.

According to spc4r30, Section 7.5.4, BLOCK DESCRIPTOR for MODE_SENSE_10
starts at byte 3, and BLOCK_DESCRIPTOR for MODE_SENSE (6) starts at byte 2.

(roland: add MODE DATA LENGTH + MEDIUM TYPE offset comment)

Cc: Roland Dreier <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Douglas Gilbert <[email protected]>
Signed-off-by: Nicholas Bellinger <[email protected]>