XFS_BROOT_SIZE_ADJ is an undocumented macro which accounts for
the difference in size between the on-disk and in-core btree
root.  It's much clearer to just use the newly-added
XFS_BMAP_BMDR_SPACE macro which gives us the on-disk size
directly.

In one case, we must test that the if_broot exists before
applying the macro, however.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NBen Myers <bpm@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>