bpf/btf: bump BTF_KFUNC_SET_MAX_CNT

bpf/btf: bump BTF_KFUNC_SET_MAX_CNT

net/bpf/test_run.c is already presenting 20 kfuncs.
net/netfilter/nf_conntrack_bpf.c is also presenting an extra 10 kfuncs.

Given that all the kfuncs are regrouped into one unique set, having
only 2 space left prevent us to add more selftests.

Bump it to 256.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Link: https://lore.kernel.org/r/20220906151303.2780789-6-benjamin.tissoires@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>