target: Fix trailing ASCII space usage in INQUIRY vendor+model

commit ee60bddba5a5f23e39598195d944aa0eb2d455e5 upstream.

This patch fixes spc_emulate_inquiry_std() to add trailing ASCII
spaces for INQUIRY vendor + model fields following SPC-4 text:

  "ASCII data fields described as being left-aligned shall have any
   unused bytes at the end of the field (i.e., highest offset) and
   the unused bytes shall be filled with ASCII space characters (20h)."

This addresses a problem with Falconstor NSS multipathing.

Reported-by: Tomas Molota <tomas.molota@lightstorm.sk>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
[bwh: Backported to 3.2, based on Nicholas's versions for 3.0 and 3.4]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>