net: tipc: fix refcount warning in tipc_aead_encrypt

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

net: tipc: fix refcount warning in tipc_aead_encrypt

syzbot reported a refcount warning [1] caused by calling get_net() on

a network namespace that is being destroyed (refcount=0). This happens

when a TIPC discovery timer fires during network namespace cleanup.

The recently added get_net() call in commit e279024617134 ("net/tipc:

fix slab-use-after-free Read in tipc_aead_encrypt_done") attempts to

hold a reference to the network namespace. However, if the namespace

is already being destroyed, its refcount might be zero, leading to the

use-after-free warning.

Replace get_net() with maybe_get_net(), which safely checks if the

refcount is non-zero before incrementing it. If the namespace is being

destroyed, return -ENODEV early, after releasing the bearer reference.

[1]: https://lore.kernel.org/all/[email protected]/T/#m12019cf9ae77e1954f666914640efa36d52704a2