Restrict the number of LUNs to 256

[dillaman]

This is the maximum supported by Linux and defining more than 256 images results in the rbd-target-gw crashing and burning

[mikechristie]

One correction to what I wrote in the email to QE.

In older distro kernels like in RHEL 7 there is a hard coded limit of 256 in the kernel target layer. There is also a limit in some versions of rtslib.

In the current upstream kernel and rtslib we are probably limited by the system. There is a SCSI spec limit due to the LUN being limited to 8 bytes. I think in the target code though we use a uint32_t in some places so that is probably the max in theory. I think we would hit some memory or other resource limit before getting 2^32 LUNs.

[pcuzner]

There are two limits here - one is the number of storage objects defined to the target, and the other the number of SO's masked to a client. It sounds like for now we should make them the same - but the goal would be to lift the SO's per target to something like 4096