We don't drop the reference on the nfsdfs filesystem with
mntput(nn->nfsd_mnt) until nfsd_exit_net(), but that won't be called
until the nfsd module's unloaded, and we can't unload the module as long
as there's a reference on nfsdfs. So this prevents module unloading.
Fixes: 2c830dd7209b ("nfsd: persist nfsd filesystem across mounts") Reported-and-Tested-by: Luo Xiaogang <lxgrxd@163.com> Signed-off-by: J. Bruce Fields <bfields@redhat.com>