octeontx2-af: fix cn20k mailbox lifetime on repeated rvu_mbox_init()

In the Linux kernel, the following vulnerability has been resolved: octeontx2-af: fix cn20k mailbox lifetime on repeated rvu_mbox_init() rvu_mbox_init() is called separately for AF-PF mailboxes during probe and for AF-VF mailboxes when SR-IOV is enabled. Each call used to allocate a new ng_rvu object, leaking the first allocation when the pointer was overwritten on the second call. Sharing one ng_rvu across both paths exposed several teardown bugs: the error path freed all cn20k mailbox DMA and kfree()d ng_rvu even when only the failing init type should be unwound, leaving live AF-PF mailbox memory in use after an AF-VF init failure. mutex_init() was also re-run on the AF-VF path while AF-PF mailbox handlers could still hold rvu->mbox_lock. Probe and SR-IOV failure paths did not release cn20k mailbox DMA either, since cleanup only happened in rvu_remove(). Allocate ng_rvu once with devm_kzalloc(), initialize mbox_lock in the same block, unwind only the mailbox memory for the failing init type, and free cn20k mailbox DMA from the probe and pci_enable_sriov() error paths.