xfs_repair: refactor attr root block pointer check

xfs_repair: refactor attr root block pointer check

In process_longform_attr, replace the agcount check with a call to the
fsblock verification function in libxfs.  Now we can also catch blocks
that point to static FS metadata.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
[sandeen: update error message]
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>