xfsprogs: Support new AGFL format

xfsprogs: Support new AGFL format

With the addition of CRCs to the filesystem format, the AGFL has a
new format structure definition. Existing code that pulls freelist
blocks out via dereferencing agfl->agfl_bno no longer works as the
location of the free list is now variable depending on the disk
format in use.

Hence all the users of agfl_bno need ot be converted to extract the
location of the first free list entry from the AGFL and grab entries
relative to that first entry. It's a simple change, but needs to be
made in several places as there is very little code reuse within and
between the different utilities in xfsprogs.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>