fsal_filesystem: uses dev_t as the fsid when get_posix_fsid() found (0,0) as fsid for a fileystem

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>
FSAL_LUSTRE/fsal_file_system: adds what's needed to manage Lustre with fsal_filesystem

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>