ceph: make ceph_netfs_read_ops static

The sparse tool complains as follows:

fs/ceph/addr.c:316:37: warning:
 symbol 'ceph_netfs_read_ops' was not declared. Should it be static?

This symbol is not used outside of addr.c, so mark it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>