tst_net.sh: Print netstress.c logs on unexpected error

When there is an unexpected error, logs should be printed as well to
know the reason:

    tcp_fastopen_run 1 TINFO: run server 'netstress -t 0 -d ltp_ns_veth1 -R 3 \
	-B /tmp/LTP_tcp_fastopen_run.TpwSlSogBJ'
    tcp_fastopen_run 1 TINFO: run client 'netstress -l -t 0 -H 10.0.0.1 -d ltp_ns_veth2 \
	-a 2 -r 2000 -c /tmp/LTP_tcp_fastopen_run.TpwSlSogBJ/tst_netload.res' 5 times
    tcp_fastopen_run 1 TWARN: netstress failed, ret: 6
    tcp_fastopen_run 1 TPASS: netstress passed, median time 123 ms, data: 118 124 122 125

While at it, factor out logging code into new function
tst_netload_print_log().

Link: https://lore.kernel.org/ltp/20260127131427.146592-1-pvorel@suse.cz/
Fixes: da16b66 ("lib/tst_net: calc mean in tst_netload()")
Reviewed-by: Li Wang <liwang@redhat.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>
Signed-off-by: Petr Vorel <pvorel@suse.cz>

Author: pevik

testcases/lib/tst_net.sh

@@ -732,10 +732,15 @@ tst_wait_ipv6_dad()
 	done
 }
 
-tst_netload_brk()
+tst_netload_print_log()
 {
 	tst_rhost_run -c "cat $TST_TMPDIR/netstress.log"
 	cat tst_netload.log
+}
+
+tst_netload_brk()
+{
+	tst_netload_print_log
 	tst_brk_ $1 $2
 }
 
@@ -853,6 +858,7 @@ tst_netload()
 				tst_netload_brk TFAIL "expected '$expect_res' but ret: '$ret'"
 
 			tst_res_ TWARN "netstress failed, ret: $ret"
+			tst_netload_print_log
 			was_failure=1
 			continue
 		fi