target: Fix REPORT TARGET PORT GROUPS handling with small allocation length

This patch fixes a bug with the handling of REPORT TARGET PORT GROUPS
containing a smaller allocation length than the payload requires causing
memory writes beyond the end of the buffer.  This patch checks for the
minimum 4 byte length for the response payload length, and also checks
upon each loop of T10_ALUA(su_dev)->tg_pt_gps_list to ensure the Target
port group and Target port descriptor list is able to fit into the
remaining allocation length.

If the response payload exceeds the allocation length length, then rd_len
is still increments to indicate to the initiator that the payload has
been truncated.

Reported-by: Roland Dreier <roland@purestorage.com>
Cc: stable@kernel.org
Signed-off-by: Nicholas Bellinger <nab@risingtidesystems.com>