net: mana: restore the XDP program pointer when pre-allocation fails

In the Linux kernel, the following vulnerability has been resolved:

net: mana: restore the XDP program pointer when pre-allocation fails

mana_xdp_set() publishes the new program into apc->bpf_prog before it
allocates anything, because mana_pre_alloc_rxbufs() sizes the buffers
from it via mana_get_rxbuf_cfg(). When that allocation fails the
function returns the error directly, skipping the err_dealloc_rxbuffs
label which is the only place that restores the previous pointer.

The attach is reported as failed, so the BPF core drops the reference it
held for the caller and the program can be freed, while apc->bpf_prog
still points at it. The next consumer of mana_xdp_get() - typically
mana_chn_setxdp() from mana_alloc_queues() on the following ifup, or
after a TX timeout reset - then calls bpf_prog_add() on freed memory.

This is reachable from an ordinary "ip link set dev ethX xdp obj ..."
whenever the per-queue RX buffer pre-allocation cannot be satisfied.

Restore the previous program on that error path.