selftests/bpf: Fix check_mtu using wrong variable type

Dan Carpenter found via Smatch static checker, that unsigned 'mtu_lo' is
never less than zero.

Variable mtu_lo should have been an 'int', because read_mtu_device_lo()
uses minus as error indications.

Fixes: b62eba563229 ("selftests/bpf: Tests using bpf_check_mtu BPF-helper")
Reported-by: Dan Carpenter <[e-mail]>
Signed-off-by: Jesper Dangaard Brouer <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://lore.kernel.org/bpf/168605104733.3636467.17945947801753092590.stgit@firesoul

tools/testing/selftests/bpf/prog_tests/check_mtu.c

···
 
 void serial_test_check_mtu(void)
 {
-	__u32 mtu_lo;
+	int mtu_lo;
 
 	if (test__start_subtest("bpf_check_mtu XDP-attach"))
 		test_check_mtu_xdp_attach();