target: Fix handling of small allocation lengths in REPORT LUNS

REPORT LUNS should not fail just because the allocation length is less
than 16.  The relevant section of SPC-4 is:

  4.2.5.6 Allocation length

  The ALLOCATION LENGTH field specifies the maximum number of bytes or
  blocks that an application client has allocated in the Data-In
  Buffer. The ALLOCATION LENGTH field specifies bytes unless a
  different requirement is stated in the command definition.

  An allocation length of zero specifies that no data shall be
  transferred. This condition shall not be considered an error.

So we should just truncate our response rather than return an error.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Spencer Baugh <sbaugh@catern.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>