bpf: update the comment for BTF_FIELDS_MAX

The commit d56b63cf

 ("bpf: add support for bpf_wq user type")
changes the fields support number to 11, just sync the comment.

Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240424054526.8031-1-haiyue.wang@intel.com


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