[XFS] stop using xfs_itobp in xfs_bulkstat

xfs_bulkstat only wants the dinode, offset and buffer from a given inode
number. Instead of using xfs_itobp on a fake inode which is complicated
and currently leads to leaks of the security data just use xfs_inotobp
which is designed to do exactly the kind of lookup xfs_bulkstat wants. The
only thing that's missing in xfs_inotobp is a flags paramter that let's us
pass down XFS_IMAP_BULKSTAT, but that can easily added.

SGI-PV: 987246

SGI-Modid: xfs-linux-melb:xfs-kern:32397a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>