xfs: fix computation of inode btree maxlevels

Commit 88740da18[1] introduced a function to compute the maximum
height of the inode btree back in 1994.  Back then, apparently, the
freespace and inode btrees shared the same geometry; however, it has
long since been the case that the inode and freespace btrees have
different record and key sizes.  Therefore, we must use m_inobt_mnr if
we want a correct calculation/log reservation/etc.

(Yes, this bug has been around for 21 years and ten months.)

(Yes, I was in middle school when this bug was committed.)

[1] http://oss.sgi.com/cgi-bin/gitweb.cgi?p=archive/xfs-import.git;a=commitdiff;h=88740da18ddd9d7ba3ebaa9502fefc6ef2fd19cd

Historical-research-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_ialloc.c

@@ -2403,8 +2403,8 @@ xfs_ialloc_compute_maxlevels(
 
 	maxleafents = (1LL << XFS_INO_AGINO_BITS(mp)) >>
 		XFS_INODES_PER_CHUNK_LOG;
-	minleafrecs = mp->m_alloc_mnr[0];
-	minnoderecs = mp->m_alloc_mnr[1];
+	minleafrecs = mp->m_inobt_mnr[0];
+	minnoderecs = mp->m_inobt_mnr[1];
 	maxblocks = (maxleafents + minleafrecs - 1) / minleafrecs;
 	for (level = 1; maxblocks > 1; level++)
 		maxblocks = (maxblocks + minnoderecs - 1) / minnoderecs;