xfs_repair: fix fino_bno calculation when rmapbt is enabled

xfs_repair: fix fino_bno calculation when rmapbt is enabled

In xfs_repair, we calculate where we think mkfs put the root inode
block.  However, the rmapbt component doesn't account for the fact
that mkfs reserved 2 AGFL blocks for the rmapbt, so its calculation
is off by a bit.  This leads to it complaining (incorrectly) about the
root inode block being in the wrong place and blowing up.

[dchinner: small comment update to indicate AGFL block accounting]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>