iscsi-target: Do not reject non-immediate CmdSNs exceeding MaxCmdSN

This patch changes iscsit_sequence_cmd() logic to no longer reject
non-immediate CmdSNs that exceed MaxCmdSN with a protocol error,
but instead silently ignore them.

This is done to correctly follow RFC-3720 Section 3.2.2.1:

   For non-immediate commands, the CmdSN field can take any
   value from ExpCmdSN to MaxCmdSN inclusive.  The target MUST silently
   ignore any non-immediate command outside of this range or non-
   immediate duplicates within the range.

Reported-by: Santosh Kulkarni <santosh.kulkarni@calsoftinc.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>