xfs_repair: fix off by one error when rebuilding high keys

xfs_repair: fix off by one error when rebuilding high keys

Fix an off-by-one error when scanning a rmap btree block for high keys
as part of rebuilding rmap btrees during phase 5.  This causes
xfs_repair to emit a corrupt filesystem, which is bad.

This can be reproduced pretty easily by exporting
TEST_XFS_REPAIR_REBUILD=1 and running generic/051 with a 1k block size.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>