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: 0ddcf43d ("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>