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.org


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