bpf: Replace kvfree with kfree for kzalloc memory

The 'backedge' pointer is allocated with kzalloc(), which returns
physically contiguous memory. Using kvfree() to deallocate such
memory is functionally safe but semantically incorrect.

Replace kvfree() with kfree() to avoid unnecessary is_vmalloc_addr()
check in kvfree().

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20250811123949.552885-1-rongqianfeng@vivo.com