xfs_repair: convert to libxfs_verify_agbno

xfs_repair: convert to libxfs_verify_agbno

Convert the homegrown verify_agbno callers to use the libxfs function,
as needed.  In some places we drop the "bno != 0" checks because those
conditionals are checking btree roots; btree roots should never be
zero if the corresponding feature bit is set; and repair skips the if
clause entirely if the feature bit is disabled.

In effect, this strengthens repair to validate that AG btree pointers
neither point to the AG headers nor past the end of the AG.

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>