FSAL_LUSTRE/fsal_file_system: adds what's needed to manage Lustre with fsal_filesystem

This commit integrates changes made after Frank and Malahal's review on github.

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>
Moves FSAL_VFS specific headers inside src/FSAL/FSAL_VFS

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>