iscsi-target: graceful disconnect on invalid mapping to iovec

Ensures the first page entry is within bounds. A failed check would
terminate the iSCSI connection instead of causing a NULL-dereference.

This violation seems to happen with certain iSCSI commands where the
computed CDB length is zero but the expected transfer length is
non-zero. The real problem is probably on the iSCSI initiator side
since there is a discrepancy between the iSCSI header and the
encapsulated CDB Opcode.

Signed-off-by: Imran Haider <imran1008@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>