bpf, sockmap: Test progs verifier error with latest clang

With a relatively recent clang (7090c10273119) and with this commit
to fix warnings in selftests (c8ed6685) that uses __sink(err)
to resolve unused variables. We get the following verifier error.

root@6e731a24b33a:/host/tools/testing/selftests/bpf# ./test_sockmap
libbpf: prog 'bpf_sockmap': BPF program load failed: Permission denied
libbpf: prog 'bpf_sockmap': -- BEGIN PROG LOAD LOG --
0: R1=ctx(off=0,imm=0) R10=fp0
; op = (int) skops->op;
0: (61) r2 = *(u32 *)(r1 +0)          ; R1=ctx(off=0,imm=0) R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
; switch (op) {
1: (16) if w2 == 0x4 goto pc+5        ; R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
2: (56) if w2 != 0x5 goto pc+15       ; R2_w=5
; lport = skops->local_port;
3: (61) r2 = *(u32 *)(r1 +68)         ; R1=ctx(off=0,imm=0) R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
; if (lport == 10000) {
4: (56) if w2 != 0x2710 goto pc+13 18: R1=ctx(off=0,imm=0) R2=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
; __sink(err);
18: (bc) w1 = w0
R0 !read_ok
processed 18 insns (limit 1000000) max_states_per_insn 0 total_states 2 peak_states 2 mark_read 1
-- END PROG LOAD LOG --
libbpf: prog 'bpf_sockmap': failed to load: -13
libbpf: failed to load object 'test_sockmap_kern.bpf.o'
load_bpf_file: (-1) No such file or directory
ERROR: (-1) load bpf failed
libbpf: prog 'bpf_sockmap': BPF program load failed: Permission denied
libbpf: prog 'bpf_sockmap': -- BEGIN PROG LOAD LOG --
0: R1=ctx(off=0,imm=0) R10=fp0
; op = (int) skops->op;
0: (61) r2 = *(u32 *)(r1 +0)          ; R1=ctx(off=0,imm=0) R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
; switch (op) {
1: (16) if w2 == 0x4 goto pc+5        ; R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
2: (56) if w2 != 0x5 goto pc+15       ; R2_w=5
; lport = skops->local_port;
3: (61) r2 = *(u32 *)(r1 +68)         ; R1=ctx(off=0,imm=0) R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
; if (lport == 10000) {
4: (56) if w2 != 0x2710 goto pc+13 18: R1=ctx(off=0,imm=0) R2=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
; __sink(err);
18: (bc) w1 = w0
R0 !read_ok
processed 18 insns (limit 1000000) max_states_per_insn 0 total_states 2 peak_states 2 mark_read 1
-- END PROG LOAD LOG --
libbpf: prog 'bpf_sockmap': failed to load: -13
libbpf: failed to load object 'test_sockhash_kern.bpf.o'
load_bpf_file: (-1) No such file or directory
ERROR: (-1) load bpf failed
libbpf: prog 'bpf_sockmap': BPF program load failed: Permission denied
libbpf: prog 'bpf_sockmap': -- BEGIN PROG LOAD LOG --
0: R1=ctx(off=0,imm=0) R10=fp0
; op = (int) skops->op;
0: (61) r2 = *(u32 *)(r1 +0)          ; R1=ctx(off=0,imm=0) R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
; switch (op) {
1: (16) if w2 == 0x4 goto pc+5        ; R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
2: (56) if w2 != 0x5 goto pc+15       ; R2_w=5
; lport = skops->local_port;
3: (61) r2 = *(u32 *)(r1 +68)         ; R1=ctx(off=0,imm=0) R2_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
; if (lport == 10000) {
4: (56) if w2 != 0x2710 goto pc+13 18: R1=ctx(off=0,imm=0) R2=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
; __sink(err);
18: (bc) w1 = w0
R0 !read_ok
processed 18 insns (limit 1000000) max_states_per_insn 0 total_states 2 peak_states 2 mark_read 1
-- END PROG LOAD LOG --

To fix simply remove the err value because its not actually used anywhere
in the testing. We can investigate the root cause later. Future patch should
probably actually test the err value as well. Although if the map updates
fail they will get caught eventually by userspace.

Fixes: c8ed6685 ("selftests/bpf: fix lots of silly mistakes pointed out by compiler")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20230523025618.113937-15-john.fastabend@gmail.com