Drop fuse.Attr return from Create, Mkdir, Mknod, Symlink, Link return.

This saves one alloc per request.

Dropping inodes is expensive (it looks through the entire tree), while
opendir/readdir/releasedir should be cheap.

call.

Fix bug in MemNodeFs.

This lowers memory pressure on concurrent loads of a filesystem that
has external latency (eg. network roundtrips).