mountd: fix checking for errors when exporting filesystems

commit 5604b35a61e22930873ffc4e9971002f578e7978
  nfs-utils: Increase the stdio file buffer size for procfs files

changed writes to some sysfs files to be line buffered (_IOLBF) where
they weren't before.  While this probably makes sense, it introduced a
bug.

With fully buffered streams, you don't expect to get an error until you
call fflush(). With line buffered streams you can get the error
from fprintf() et al.

qword_eol() only tests the return from fflush(), not from fprintf().
Consequently errors were not noticed.

One result of this is that if you export, with crossmnt, a filesystem
underneath which are mounted non-exportable filesystems (e.g. /proc)
then an 'ls -l' on the client will block indefinitely waiting for a
meaningful 'yes' or 'no' from the server, but will never get one.

This patch changes qword_eol to test both fprintf and fflush.

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>