NFSD: Mark filecache "down" if init fails

NFSD: Mark filecache "down" if init fails

NeilBrown says:
> The handling of NFSD_FILE_CACHE_UP is strange.  nfsd_file_cache_init()
> sets it, but doesn't clear it on failure.  So if nfsd_file_cache_init()
> fails for some reason, nfsd_file_cache_shutdown() would still try to
> clean up if it was called.

Reported-by: NeilBrown <neilb@suse.de>
Fixes: c7b824c3d06c ("NFSD: Replace the "init once" mechanism")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>