rpc.nfsd: mount up nfsdfs is it doesn't appear to be mounted yet

There's a bit of a chicken and egg problem when nfsd is run the first
time. On Fedora/RHEL at least, /proc/fs/nfsd is mounted up whenever nfsd
is plugged in via a modprobe.conf "install" directive.

If someone runs rpc.nfsd without plugging in nfsd.ko first,
/proc/fs/nfsd won't be mounted and rpc.nfsd will end up using the legacy
nfsctl interface. After that, nfsd will be plugged in and subsequent
rpc.nfsd invocations will use that instead.

This is a problem as some nfsd command-line options are ignored when the
legacy interface is used. It'll also be a problem for people who want
IPv6 enabled servers. The upshot is that we really don't want to use the
legacy interface unless there is no other option.

To avoid this situation, have rpc.nfsd check to see if the "threads"
file is already present. If it's not, then make an attempt to mount
/proc/fs/nfsd.  This is a "best-effort" sort of thing, however so we
just ignore the return code from the mount attempt and fall back to
using nfsctl() if it fails.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>

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