selftests/bpf: Fix compilation errors: Assign a value to a constant

Commit bc292ab00f6c("mm: introduce vma->vm_flags wrapper functions")
turns the vm_flags into a const variable.

Added bpf_find_vma test in commit f108662b27c9("selftests/bpf: Add tests
for bpf_find_vma") to assign values to variables that declare const in
find_vma_fail1.c programs, which is an error to the compiler and does not
test BPF verifiers. It is better to replace 'const vm_flags_t vm_flags'
with 'unsigned long vm_start' for testing.

$ make -C tools/testing/selftests/bpf/ -j8
...
progs/find_vma_fail1.c:16:16: error: cannot assign to non-static data
member 'vm_flags' with const-qualified type 'const vm_flags_t' (aka
'const unsigned long')
vma->vm_flags |= 0x55;
~~~~~~~~~~~~~ ^
../tools/testing/selftests/bpf/tools/include/vmlinux.h:1898:20:
note: non-static data member 'vm_flags' declared const here
const vm_flags_t vm_flags;
~~~~~~~~~~~`~~~~~~^~~~~~~~

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/tencent_CB281722B3C1BD504C16CDE586CACC2BE706@qq.com

selftests/bpf: Fix progs/find_vma_fail1.c build error.

The commit 11e456cae91e ("selftests/bpf: Fix compilation errors: Assign a value to a constant")
fixed the issue cleanly in bpf-next.
This is an alternative fix in bpf tree to avoid merge conflict between bpf and bpf-next.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add tests for bpf_find_vma

Add tests for bpf_find_vma in perf_event program and kprobe program. The
perf_event program is triggered from NMI context, so the second call of
bpf_find_vma() will return -EBUSY (irq_work busy). The kprobe program,
on the other hand, does not have this constraint.

Also add tests for illegal writes to task or vma from the callback
function. The verifier should reject both cases.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211105232330.1936330-3-songliubraving@fb.com