bpf: Allow fall back to interpreter for programs with stack size <= 512

OpenWRT users reported regression on ARMv6 devices after updating to latest
HEAD, where tcpdump filter:

tcpdump "not ether host 3c37121a2b3c and not ether host 184ecbca2a3a \
and not ether host 14130b4d3f47 and not ether host f0f61cf440b7 \
and not ether host a84b4dedf471 and not ether host d022be17e1d7 \
and not ether host 5c497967208b and not ether host 706655784d5b"

fails with warning: "Kernel filter failed: No error information"
when using config:
 # CONFIG_BPF_JIT_ALWAYS_ON is not set
 CONFIG_BPF_JIT_DEFAULT_ON=y

The issue arises because commits:
1. "bpf: Fix array bounds error with may_goto" changed default runtime to
   __bpf_prog_ret0_warn when jit_requested = 1
2. "bpf: Avoid __bpf_prog_ret0_warn when jit fails" returns error when
   jit_requested = 1 but jit fails

This change restores interpreter fallback capability for BPF programs with
stack size <= 512 bytes when jit fails.

Reported-by: Felix Fietkau <nbd@nbd.name>
Closes: https://lore.kernel.org/bpf/2e267b4b-0540-45d8-9310-e127bf95fc63@nbd.name/


Fixes: 6ebc5030 ("bpf: Fix array bounds error with may_goto")
Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250909144614.2991253-1-kafai.wan@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>