selftests/bpf: Fix error checking on reading the tcp_fastopen sysctl

There is a typo in checking the "saved_tcp_fo" and instead
"saved_tcp_syncookie" is checked again.  This patch fixes it
and also breaks them into separate if statements such that
the test will abort asap.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200211175910.3235321-1-kafai@fb.com