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

Commit bc292ab0("mm: introduce vma->vm_flags wrapper functions")
turns the vm_flags into a const variable.

Added bpf_find_vma test in commit f108662b("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

tools/testing/selftests/bpf/progs/find_vma_fail1.c

@@ -13,7 +13,7 @@ static long write_vma(struct task_struct *task, struct vm_area_struct *vma,
 		      struct callback_ctx *data)
 {
 	/* writing to vma, which is illegal */
-	vma->vm_flags |= 0x55;
+	vma->vm_start = 0xffffffffff600000;
 
 	return 0;
 }