fuse: clean up args in fuse_finish_open() and fuse_release_fill()

Move setting ff->fh, ff->nodeid and file->private_data outside
fuse_finish_open().  Add ->open_flags member to struct fuse_file.

This simplifies the argument passing to fuse_finish_open() and
fuse_release_fill(), and paves the way for creating an open helper
that doesn't need an inode pointer.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>