xfs: fix maxicount division by zero error

In xfs_ialloc_setup_geometry, it's possible for a malicious/corrupt fs
image to set an unreasonably large value for sb_inopblog which will
cause ialloc_blks to be zero.  If sb_imax_pct is also set, this results
in a division by zero error in the second do_div call.  Therefore, force
maxicount to zero if ialloc_blks is zero.

Note that the kernel metadata verifiers will catch the garbage inopblog
value and abort the fs mount long before it tries to set up the inode
geometry; this is needed to avoid a crash in xfs_db while setting up the
xfs_mount structure.

Found by fuzzing sb_inopblog to 122 in xfs/350.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>

fs/xfs/libxfs/xfs_ialloc.c

@@ -2787,8 +2787,13 @@ xfs_ialloc_setup_geometry(
 	igeo->inobt_maxlevels = xfs_btree_compute_maxlevels(igeo->inobt_mnr,
 			inodes);
 
-	/* Set the maximum inode count for this filesystem. */
-	if (sbp->sb_imax_pct) {
+	/*
+	 * Set the maximum inode count for this filesystem, being careful not
+	 * to use obviously garbage sb_inopblog/sb_inopblock values.  Regular
+	 * users should never get here due to failing sb verification, but
+	 * certain users (xfs_db) need to be usable even with corrupt metadata.
+	 */
+	if (sbp->sb_imax_pct && igeo->ialloc_blks) {
 		/*
 		 * Make sure the maximum inode count is a multiple
 		 * of the units we allocate inodes in.