selftests/bpf: Add torn write detection test for htab BPF_F_LOCK

selftests/bpf: Add torn write detection test for htab BPF_F_LOCK

Add a consistency subtest to htab_reuse that detects torn writes
caused by the BPF_F_LOCK lockless update racing with element
reallocation in alloc_htab_elem().

The test uses three thread roles started simultaneously via a pipe:
 - locked updaters: BPF_F_LOCK|BPF_EXIST in-place updates
 - delete+update workers: delete then BPF_ANY|BPF_F_LOCK insert
 - locked readers: BPF_F_LOCK lookup checking value consistency

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260401-bpf_map_torn_writes-v1-2-782d071c55e7@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>