fuse: hold inode instead of path after release

path_put() in release could trigger a DESTROY request in fuseblk.  The
possible deadlock was worked around by doing the path_put() with
schedule_work().

This complexity isn't needed if we just hold the inode instead of the path.
Since we now flush all requests before destroying the super block we can be
sure that all held inodes will be dropped.

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