[iscsi] Ensure ISID is consistent within an iSCSI session

[iscsi] Ensure ISID is consistent within an iSCSI session

Commit 5f4ab0d ("[iscsi] Randomise a portion of the ISID to force new
session instantiation") introduced a regression by randomising the
ISID on each call to iscsi_start_login(), which may be called more
than once per connection, rather than on each call to
iscsi_open_connection(), which is guaranteed to be called only once
per connection.  This is incorrect behaviour that causes our
connection to be rejected by some iSCSI targets (observed with a
COMSTAR target under OpenSolaris).

Fix by generating the ISID in iscsi_open_connection(), and storing the
randomised ISID as part of the session state.

Signed-off-by: Michael Brown <mcb30@ipxe.org>