nfsd: Ensure knfsd shuts down when the "nfsd" pseudofs is unmounted

nfsd: Ensure knfsd shuts down when the "nfsd" pseudofs is unmounted

commit 345d8ceeff3efa28a0e171b085ba0b8c6e106b16 upstream.

In order to ensure that knfsd threads don't linger once the nfsd
pseudofs is unmounted (e.g. when the container is killed) we let
nfsd_umount() shut down those threads and wait for them to exit.

This also should ensure that we don't need to do a kernel mount of
the pseudofs, since the thread lifetime is now limited by the
lifetime of the filesystem.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>