fuse: clean up open file passing in setattr

Clean up supplying open file to the setattr operation.  In addition to being a
cleanup it prepares for the changes in the way the open file is passed to the
setattr method.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>