Decrement the number of elements in the map in case the allocation
of a new node fails.

Fixes: 6c905981 ("bpf: pre-allocate hash map elements")
Signed-off-by: NMauricio Vasquez B <mauricio.vasquez@polito.it>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>