[XFS] xfs_bmap_compute_maxlevels should be based on di_forkoff

Fix up xfs_bmap_compute_maxlevels() to account for the case when we go
from using attr2 to using attr1. In that case attr1 will no longer
necessarily be at m_attr_offset>>3, but could be at a different value for
di_forkoff. Therefore, we return the worst case scenario using MINDBTPTRS
and MINABTPTRS, as this function is used for determining the maximum log
space.

SGI-PV: 979606
SGI-Modid: xfs-linux-melb:xfs-kern:30862a

Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>