selftests/bpf: Workaround strict bpf_lsm return value check.

test_progs-no_alu32 -t libbpf_get_fd_by_id_opts
is being rejected by the verifier with the following error
due to compiler optimization:

6: (67) r0 <<= 62 ; R0_w=scalar(smax=0x4000000000000000,umax=0xc000000000000000,smin32=0,smax32=umax32=0,var_off=(0x0; 0xc000000000000000))
7: (c7) r0 s>>= 63 ; R0_w=scalar(smin=smin32=-1,smax=smax32=0)
; @ test_libbpf_get_fd_by_id_opts.c:0
8: (57) r0 &= -13 ; R0_w=scalar(smax=0x7ffffffffffffff3,umax=0xfffffffffffffff3,smax32=0x7ffffff3,umax32=0xfffffff3,var_off=(0x0; 0xfffffffffffffff3))
; int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) @ test_libbpf_get_fd_by_id_opts.c:27
9: (95) exit
At program exit the register R0 has smax=9223372036854775795 should have been in [-4095, 0]

Workaround by adding barrier().
Eventually the verifier will be able to recognize it.

Fixes: 5d99e198be27 ("bpf, lsm: Add check for BPF LSM return value")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>