nfsd4: fix nfsdfs reference count loop

nfsd4: fix nfsdfs reference count loop

BugLink: https://bugs.launchpad.net/bugs/1886995
[ Upstream commit 681370f4b00af0fcc65bbfb9f82de526ab7ceb0a ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>