xfs_db: dump per-AG reservations

xfs_db: dump per-AG reservations

Add a new 'agresv' command to print the size and free blocks count of an
AG along with the size and usage of the per-AG reservation.  This
command can be used to aid in diagnosing why a particular filesystem
fails the mount time per-AG space reservation, and to figure out how
much space needs to be freed from a given AG to fix the problem.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>