selftests/bpf: add a selftest with __user tag

Added a selftest with three__user usages: a __user pointer-type argument
in bpf_testmod, a __user pointer-type struct member in bpf_testmod,
and a __user pointer-type struct member in vmlinux. In all cases,
directly accessing the user memory will result verification failure.

$ ./test_progs -v -n 22/3
...
libbpf: prog 'test_user1': BPF program load failed: Permission denied
libbpf: prog 'test_user1': -- BEGIN PROG LOAD LOG --
R1 type=ctx expected=fp
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
; int BPF_PROG(test_user1, struct bpf_testmod_btf_type_tag_1 *arg)
0: (79) r1 = *(u64 *)(r1 +0)
func 'bpf_testmod_test_btf_type_tag_user_1' arg0 has btf_id 136561 type STRUCT 'bpf_testmod_btf_type_tag_1'
1: R1_w=user_ptr_bpf_testmod_btf_type_tag_1(id=0,off=0,imm=0)
; g = arg->a;
1: (61) r1 = *(u32 *)(r1 +0)
R1 invalid mem access 'user_ptr_'
...
#22/3 btf_tag/btf_type_tag_user_mod1:OK

$ ./test_progs -v -n 22/4
...
libbpf: prog 'test_user2': BPF program load failed: Permission denied
libbpf: prog 'test_user2': -- BEGIN PROG LOAD LOG --
R1 type=ctx expected=fp
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
; int BPF_PROG(test_user2, struct bpf_testmod_btf_type_tag_2 *arg)
0: (79) r1 = *(u64 *)(r1 +0)
func 'bpf_testmod_test_btf_type_tag_user_2' arg0 has btf_id 136563 type STRUCT 'bpf_testmod_btf_type_tag_2'
1: R1_w=ptr_bpf_testmod_btf_type_tag_2(id=0,off=0,imm=0)
; g = arg->p->a;
1: (79) r1 = *(u64 *)(r1 +0) ; R1_w=user_ptr_bpf_testmod_btf_type_tag_1(id=0,off=0,imm=0)
; g = arg->p->a;
2: (61) r1 = *(u32 *)(r1 +0)
R1 invalid mem access 'user_ptr_'
...
#22/4 btf_tag/btf_type_tag_user_mod2:OK

$ ./test_progs -v -n 22/5
...
libbpf: prog 'test_sys_getsockname': BPF program load failed: Permission denied
libbpf: prog 'test_sys_getsockname': -- BEGIN PROG LOAD LOG --
R1 type=ctx expected=fp
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
; int BPF_PROG(test_sys_getsockname, int fd, struct sockaddr *usockaddr,
0: (79) r1 = *(u64 *)(r1 +8)
func '__sys_getsockname' arg1 has btf_id 2319 type STRUCT 'sockaddr'
1: R1_w=user_ptr_sockaddr(id=0,off=0,imm=0)
; g = usockaddr->sa_family;
1: (69) r1 = *(u16 *)(r1 +0)
R1 invalid mem access 'user_ptr_'
...
#22/5 btf_tag/btf_type_tag_user_vmlinux:OK

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220127154616.659314-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>