CVE-2025-38273: 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 tipcaeadencrypt

syzbot reported a refcount warning [1] caused by calling getnet() 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 getnet() call in commit e279024617134 ("net/tipc: fix slab-use-after-free Read in tipcaeadencryptdone") 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 getnet() with maybegetnet(), 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/68342b55.a70a0220.253bc2.0091.GAE@google.com/T/#m12019cf9ae77e1954f666914640efa36d52704a2