bpf: Remove WARN_ON_ONCE in check_ids()

check_ids() warned when it ran out of idmap slots, assuming this was
impossible because the slots are bounded by the number of registers and
stack slots. That assumption no longer holds: referenced dynptrs acquire
an intermediate reference that lives in refs[] but is not backed by any
register or stack slot [0], so a program can accumulate more reference
ids than the idmap can hold and exhaust it.

Exhaustion is fine for verification correctness. check_ids() already
returns false, which makes the states compare as not equivalent and
prevents unsound pruning. The only effect of the WARN_ON_ONCE() is log
noise, or a panic under panic_on_warn. Drop the warning and keep
returning false.

[0] 308c7a0ae885 ("bpf: Refactor object relationship tracking and fix dynptr UAF bug")

Signed-off-by: Amery Hung <redacted>

This removes a WARN_ON_ONCE() that can panic under panic_on_warn, and
the commit body explains the idmap exhaustion only became reachable once
referenced dynptrs began acquiring an intermediate reference in refs[]
that is not backed by a register or stack slot.

Should this carry a Fixes: tag pointing at the commit that introduced
that behavior, which the body already cites as [0]?

  Fixes: 308c7a0ae885 ("bpf: Refactor object relationship tracking and fix dynptr UAF bug")

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