dibs: loopback: validate offset and size in move_data()

In the Linux kernel, the following vulnerability has been resolved: dibs: loopback: validate offset and size in move_data() The loopback move_data() performs a memcpy into the registered DMB without checking whether offset + size exceeds the DMB length. Unlike real ISM hardware, which enforces memory region bounds natively, the software loopback has no such protection. A peer-supplied out-of-bounds offset or oversized write would result in an OOB write past the allocated kernel buffer. Add an explicit bounds check before the memcpy to reject such requests with -EINVAL.