iscsi-target: Fix-up all zero data-length CDBs with R/W_BIT set

This patch changes special case handling for ISCSI_OP_SCSI_CMD
where an initiator sends a zero length Expected Data Transfer
Length (EDTL), but still sets the WRITE and/or READ flag bits
when no payload transfer is requested.

Many, many moons ago two special cases where added for an ancient
version of ESX that has long since been fixed, so instead of adding
a new special case for the reported bug with a Broadcom 57800 NIC,
go ahead and always strip off the incorrect WRITE + READ flag bits.

Also, avoid sending a reject here, as RFC-3720 does mandate this
case be handled without protocol error.

Reported-by: Witold Bazakbal <865perl@wp.pl>
Tested-by: Witold Bazakbal <865perl@wp.pl>
Cc: <stable@vger.kernel.org> #3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>