selftests: Make sure all 'skel' variables are declared static

If programs in prog_tests using skeletons declare the 'skel' variable as
global but not static, that will lead to linker errors on the final link of
the prog_tests binary due to duplicate symbols. Fix a few instances of this.

Fixes: b18c1f0aa4 ("bpf: selftest: Adapt sock_fields test to use skel and global variables")
Fixes: 9a856cae22 ("bpf: selftest: Add test_btf_skc_cls_ingress")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200929123026.46751-1-toke@redhat.com

tools/testing/selftests/bpf/prog_tests/btf_skc_cls_ingress.c

@@ -16,7 +16,7 @@
 #include "test_progs.h"
 #include "test_btf_skc_cls_ingress.skel.h"
 
-struct test_btf_skc_cls_ingress *skel;
+static struct test_btf_skc_cls_ingress *skel;
 struct sockaddr_in6 srv_sa6;
 static __u32 duration;
 

tools/testing/selftests/bpf/prog_tests/sock_fields.c

@@ -36,7 +36,7 @@ struct bpf_spinlock_cnt {
 
 static struct sockaddr_in6 srv_sa6, cli_sa6;
 static int sk_pkt_out_cnt10_fd;
-struct test_sock_fields *skel;
+static struct test_sock_fields *skel;
 static int sk_pkt_out_cnt_fd;
 static __u64 parent_cg_id;
 static __u64 child_cg_id;