As of kernel 4.11, the LIO target will propose a value for
FirstBurstLength if the initiator did not do so.  This is entirely
redundant in our case, since FirstBurstLength is defined by RFC 3720
to be

  "Irrelevant when: ( InitialR2T=Yes and ImmediateData=No )"

and we already enforce both InitialR2T=Yes and ImmediateData=No in our
initial proposal.  However, LIO (arguably correctly) complains when we
do not respond to its redundant proposal of an already-irrelevant
value.

Fix by always proposing the default value for FirstBurstLength.

Debugged-by: Patrick Seeburger <info@8bit.de>
Tested-by: Patrick Seeburger <info@8bit.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>