This adds reader's lock for the_nilfs->cno in nilfs_ioctl_sync,
for the_nilfs->cno should be proctected by segctor_sem when reading.
Signed-off-by: NJiro SEKIBA <jir@unicus.jp>
Signed-off-by: NRyusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>