sockfd_cache: guard fds_count with sockfd_cache.lock

Otherwise, fds_count can increase while sockfd_cache_add() initializes
new->fds[i].fd, and it leads to severe memory corruption.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>