"seeking" works by skipping forward to the requested offset,
first rewinding to zero if seeking back.

The performance is obviously bad, however, seeking
inside a directory does not seem to be too common.

I think the solution is good enough until we see a
use case where this causes performance issues.

Fixes https://github.com/hanwen/go-fuse/issues/344

Change-Id: Ia7f7fbffaf932c16919eafede6b70f4aff245c25