nfsd: fail init on /proc/fs/nfs/exports creation failure

I assume the reason failure of creation was ignored here was just to
continue support embedded systems that want nfsd but not proc.

However, in cases where proc is supported it would be clearer to fail
entirely than to come up with some features disabled.

Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>