bpf: replace pop/push emptiness check with bpf_list_empty()

bpf: replace pop/push emptiness check with bpf_list_empty()

Simplify fq_flows_is_empty() by replacing the pop/push based emptiness
check with a direct call to bpf_list_empty().
This avoids unnecessary list mutation and simplifies the code while
preserving correctness.

Signed-off-by: Suchit Karunakaran <suchitkarunakaran@gmail.com>
Changes since v1:
- Removed unused variable node
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260524025853.13786-1-suchitkarunakaran@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>