dax/kmem: account for partial discontiguous resource upon removal

In the Linux kernel, the following vulnerability has been resolved: dax/kmem: account for partial discontiguous resource upon removal When dev_dax_kmem_probe() partially succeeds (at least one range is mapped) but a subsequent range fails request_mem_region() or add_memory_driver_managed(), the probe silently continues, ultimately returning success, but with the corresponding range resource NULL'ed out. dev_dax_kmem_remove() iterates over all dax_device ranges regardless of if the underlying resource exists. When remove_memory() is called later, it returns 0 because the memory was never added which causes dev_dax_kmem_remove() to incorrectly assume the (nonexistent) resource can be removed and attempts cleanup on a NULL pointer. Fix this by skipping these ranges altogether, noting that these cases are considered success, such that the cleanup is still reached when all actually-added ranges are successfully removed.