bpf: fix implicit declaration of bpf_prog_add

For the ifndef case of CONFIG_BPF_SYSCALL, an inline version of
bpf_prog_add needs to exist otherwise the build breaks on some configs.

 drivers/net/ethernet/mellanox/mlx4/en_netdev.c:2544:10: error: implicit declaration of function 'bpf_prog_add'
       prog = bpf_prog_add(prog, priv->rx_ring_num - 1);

The function is introduced in
59d3656d5bf50 ("bpf: add bpf_prog_add api for bulk prog refcnt")
and first used in
47f1afdba2b87 ("net/mlx4_en: add support for fast rx drop bpf program").

Fixes: 47f1afdba2b87 ("net/mlx4_en: add support for fast rx drop bpf program")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Tariq Toukan <ttoukan.linux@gmail.com>
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/bpf.h

@@ -289,6 +289,10 @@ static inline struct bpf_prog *bpf_prog_get_type(u32 ufd,
 {
 	return ERR_PTR(-EOPNOTSUPP);
 }
+static inline struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
 
 static inline void bpf_prog_put(struct bpf_prog *prog)
 {