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.