staging/lustre: move IFTODT/DTTOIF to lustre_idl.h

They are in fact definition of how client/server talks
to each other about file type.

Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>