selftests/bpf: Test PROBE_MEM of VSYSCALL_ADDR on x86-64

The vsyscall is a legacy API for fast execution of system calls. It maps
a page at address VSYSCALL_ADDR into the userspace program. This address
is in the top 10MB of the address space:

ffffffffff600000 - ffffffffff600fff |    4 kB | legacy vsyscall ABI

The last commit fixes the x86-64 BPF JIT to skip accessing addresses in
this memory region. Add this address to bpf_testmod_return_ptr() so we
can make sure that it is fixed.

After this change and without the previous commit, subprogs_extable
selftest will crash the kernel.
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20240424100210.11982-4-puranjay@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -205,6 +205,9 @@ __weak noinline struct file *bpf_testmod_return_ptr(int arg)
 	case 5: return (void *)~(1ull << 30);	/* trigger extable */
 	case 6: return &f;			/* valid addr */
 	case 7: return (void *)((long)&f | 1);	/* kernel tricks */
+#ifdef CONFIG_X86_64
+	case 8: return (void *)VSYSCALL_ADDR;   /* vsyscall page address */
+#endif
 	default: return NULL;
 	}
 }