bpf: fix a memory leak in the LRU and LRU_PERCPU hash maps

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

bpf: fix a memory leak in the LRU and LRU_PERCPU hash maps

The LRU and LRU_PERCPU maps allocate a new element on update before locking the

target hash table bucket. Right after that the maps try to lock the bucket.

If this fails, then maps return -EBUSY to the caller without releasing the

allocated element. This makes the element untracked: it doesn't belong to

either of free lists, and it doesn't belong to the hash table, so can't be

re-used; this eventually leads to the permanent -ENOMEM on LRU map updates,

which is unexpected. Fix this by returning the element to the local free list

if bucket locking fails.