nfsd4: be forgiving in the absence of the recovery directory

If the recovery directory doesn't exist, then behavior after a reboot
will be suboptimal.  But it's unnecessarily harsh to then prevent the
nfsv4 server from working at all.  Instead just print a warning
(already done in nfsd4_init_recdir()) and soldier on.

Tested-by: Lior <lior@tonian.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>