fuse: verify write return

commit 8aab336b upstream.

Make sure filesystem is not returning a bogus number of bytes written.

Fixes: ea9b9907 ("fuse: implement perform_write")
Cc: <stable@vger.kernel.org> # v2.6.26
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>