selftests/x86/fsgsbase: Test PTRACE_PEEKUSER for GSBASE with invalid LDT GS

This tests commit:

  8ab49526 ("x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task")

Unpatched kernels will OOPS.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/c618ae86d1f757e01b1a8e79869f553cb88acf9a.1598461151.git.luto@kernel.org