Run bpf_tcp_ca selftests (./test_progs -t bpf_tcp_ca) on a Loongarch
platform, some "Segmentation fault" errors occur:

'''
 test_dctcp:PASS:bpf_dctcp__open_and_load 0 nsec
 test_dctcp:FAIL:bpf_map__attach_struct_ops unexpected error: -524
 #29/1    bpf_tcp_ca/dctcp:FAIL
 test_cubic:PASS:bpf_cubic__open_and_load 0 nsec
 test_cubic:FAIL:bpf_map__attach_struct_ops unexpected error: -524
 #29/2    bpf_tcp_ca/cubic:FAIL
 test_dctcp_fallback:PASS:dctcp_skel 0 nsec
 test_dctcp_fallback:PASS:bpf_dctcp__load 0 nsec
 test_dctcp_fallback:FAIL:dctcp link unexpected error: -524
 #29/4    bpf_tcp_ca/dctcp_fallback:FAIL
 test_write_sk_pacing:PASS:open_and_load 0 nsec
 test_write_sk_pacing:FAIL:attach_struct_ops unexpected error: -524
 #29/6    bpf_tcp_ca/write_sk_pacing:FAIL
 test_update_ca:PASS:open 0 nsec
 test_update_ca:FAIL:attach_struct_ops unexpected error: -524
 settcpca:FAIL:setsockopt unexpected setsockopt: \
					actual -1 == expected -1
 (network_helpers.c:99: errno: No such file or directory) \
					Failed to call post_socket_cb
 start_test:FAIL:start_server_str unexpected start_server_str: \
					actual -1 == expected -1
 test_update_ca:FAIL:ca1_ca1_cnt unexpected ca1_ca1_cnt: \
					actual 0 <= expected 0
 #29/9    bpf_tcp_ca/update_ca:FAIL
 #29      bpf_tcp_ca:FAIL
 Caught signal #11!
 Stack trace:
 ./test_progs(crash_handler+0x28)[0x5555567ed91c]
 linux-vdso.so.1(__vdso_rt_sigreturn+0x0)[0x7ffffee408b0]
 ./test_progs(bpf_link__update_map+0x80)[0x555556824a78]
 ./test_progs(+0x94d68)[0x5555564c4d68]
 ./test_progs(test_bpf_tcp_ca+0xe8)[0x5555564c6a88]
 ./test_progs(+0x3bde54)[0x5555567ede54]
 ./test_progs(main+0x61c)[0x5555567efd54]
 /usr/lib64/libc.so.6(+0x22208)[0x7ffff2aaa208]
 /usr/lib64/libc.so.6(__libc_start_main+0xac)[0x7ffff2aaa30c]
 ./test_progs(_start+0x48)[0x55555646bca8]
 Segmentation fault
'''

This is because BPF trampoline is not implemented on Loongarch yet,
"link" returned by bpf_map__attach_struct_ops() is NULL. test_progs
crashs when this NULL link passes to bpf_link__update_map(). This
patch adds NULL checks for all links in bpf_tcp_ca to fix these errors.
If "link" is NULL, goto the newly added label "out" to destroy the skel.

v2:
 - use "goto out" instead of "return" as Eduard suggested.

Fixes: 06da9f3b ("selftests/bpf: Test switching TCP Congestion Control algorithms.")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/r/b4c841492bd4ed97964e4e61e92827ce51bf1dc9.1720615848.git.tanggeliang@kylinos.cnSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>