xfs_db: convert the agresv command to use for_each_perag

Convert the AG iteration loop for this debugger command to use
for_each_perag, since it's the only place in userspace that obvious
wants it.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>