ipv4: Fix memory leak in exception case for splitting tries

Fix a small memory leak that can occur where we leak a fib_alias in the
event of us not being able to insert it into the local table.

Fixes: 0ddcf43d5d4a0 ("ipv4: FIB Local/MAIN table collapse")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1743,8 +1743,10 @@ struct fib_table *fib_trie_unmerge(struct fib_table *oldtb)
 				local_l = fib_find_node(lt, &local_tp, l->key);
 
 			if (fib_insert_alias(lt, local_tp, local_l, new_fa,
-					     NULL, l->key))
+					     NULL, l->key)) {
+				kmem_cache_free(fn_alias_kmem, new_fa);
 				goto out;
+			}
 		}
 
 		/* stop loop if key wrapped back to 0 */