iscsi-target; Bump default CmdSN Depth to 64

This patch bumps the default TA_DEFAULT_CMDSN_DEPTH from 16 -> 64,
which is closer to a sane default for larger pipes @ 10 Gb/sec
with traditional iSCSI, or @ 40/56 Gb/sec Ethernet/Infiniband with
iSCSI Extentions for RDMA.

There is really no downside to increasing this default value for
1 Gb/sec.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>