selftests/pipe: add pipe_bench microbenchmark

selftests/pipe: add pipe_bench microbenchmark

Add a small selftest that stresses pipe->mutex contention by spawning N
writer threads that hammer a single pipe with multi-page writes, plus M
reader threads that drain. Each writer records its own write() latency
samples into a log2-bucketed histogram; main aggregates and prints
total writes, throughput, average and percentile (p50/p99) latencies,
and the maximum observed latency.

Pass --memory-pressure to fork stress-ng (--vm 4 --vm-bytes 80%
--vm-method all) for the duration of the run, so alloc_page() in
anon_pipe_write() routinely hits direct reclaim. The flag fails
fast if stress-ng is not on $PATH.

Program print something like the following, for different writes,
readers, msgsizes and memory pressure:

	config: writers=X readers=Y msgsize=Z duration=3 pipe_size=1048576
	memory_pressure=[no|yes]
	writes: total=54451 rate=18150/s
	throughput_MBps: 1134.40
	lat_avg_ns: 275355
	lat_p50_ns_upper: 262143
	lat_p99_ns_upper: 1048575
	lat_max_ns: 2145633

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260524-fix_pipe-v3-2-bb4a75d23a90@debian.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>