bpftool: Fix errno variable usage

The test meant to use the saved value of errno. Given the current code, it
makes no practical difference however.

Fixes: bf598a8f

 ("bpftool: Improve handling of ENOENT on map dumps")
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>