fuse: remove the unneeded result variable

Return the value fuse_dev_release() directly instead of storing it in
another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>