Push the file layout driver into a subdirectory

The object and block layouts already exist in their own
subdirectories. This patch completes the set!

Note that as a layout denotes nfs4 already, I stripped
that prefix out of the file names.

Signed-off-by: Tom Haynes <Thomas.Haynes@primarydata.com>
Acked-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>