iser-target: Remove ISER_RECV_DATA_SEG_LEN

This is the same as ISCSI_DEF_MAX_RECV_SEG_LEN (and must be the same given
the structure layouts), so just use that constant instead.  This also
allows removing ISER_RX_LOGIN_SIZE in favor of ISER_RX_PAYLOAD_SIZE.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>