bpf: btf: fix inconsistent IS_ERR and PTR_ERR

Fix inconsistent IS_ERR and PTR_ERR in get_btf,
the proper pointer to be passed as argument is '*btf'

This issue was detected with the help of Coccinelle.

Fixes: 2d3feca8 ("bpf: btf: print map dump and lookup with btf info")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>