For verification purposes, AGFLs need to be initialised to a known
set of values. For upcoming CRC changes, they are also headers that
need to be initialised. Currently, growfs does neither for the AGFLs
- it ignores them completely. Add initialisation of the AGFL to be
full of invalid block numbers (NULLAGBLOCK) to put the
infrastructure in place needed for CRC support.

Includes a comment clarification from Jeff Liu.
Signed-off-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by Rich Johnston <rjohnston@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>