iser: Fix bidirectional chap

When bidirectional chap is used, after the authentication phase is completed, the next
message from the target to the initiator should contain only the session parameters.
Currently it is wrongly prefixed with part of the previous message (which contained
authentication data). Fix that by initializing tx_pdu->membuf.size to zero before any
message is created in iser_login_exec.

Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
spc: Allow 14bit lun numbers.

Only the topmost 2 bits are reserved, and flat space addressing allows
14bit luns.
Tested by creating a target with 1500 luns.

Signed-off-by: Jan Vesely <jvesely@redhat.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>