cat: unnecessary +1 splice() call with small input

[oech3]

separated issue from #10832 .

$ head -c 4096 /dev/zero >/tmp/huge
$ strace -c -e splice cat /tmp/huge >/dev/null
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.000007           1         4         1 splice

But we should have 3 splice() as

1. try to splice() without broker pipe (FAIL)
2. splice() to broker pipe
3. splice() to output from broker

related: #11844 (comment)

[oech3]

Oh... we cannot use stat due to FUSE issue...
So this is low-priority task. (clap has more overhead)

[oech3]

We have 2 if 1st splice succeed while GNU always have 3 splice()s with 65536 byte input. We can do same things with GNU. But I think trying to save pipe is useful for throughput. Closing.