xfs: check for dinode realtime flag corruption

Ramon tested XFS with a modified version of fsfuzzer and hit a NULL
pointer dereference in __xfs_get_blocks due to the RT device target
pointer being NULL.

To fix this reject inode with the realtime bit set on a a filesystem
without an RT subvolume during inode read.

Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Eric Sandeen <[e-mail]>
Reviewed-by: Felix Blyakher <[e-mail]>
Reported-by: Ramon de Carvalho Valle <[e-mail]>
Tested-by: Ramon de Carvalho Valle <[e-mail]>
Signed-off-by: Felix Blyakher <[e-mail]>

fs/xfs/xfs_inode.c

···
 		return XFS_ERROR(EFSCORRUPTED);
 	}
 
+	if (unlikely((ip->i_d.di_flags & XFS_DIFLAG_REALTIME) &&
+		     !ip->i_mount->m_rtdev_targp)) {
+		xfs_fs_repair_cmn_err(CE_WARN, ip->i_mount,
+			"corrupt dinode %Lu, has realtime flag set.",
+			ip->i_ino);
+		XFS_CORRUPTION_ERROR("xfs_iformat(realtime)",
+				     XFS_ERRLEVEL_LOW, ip->i_mount, dip);
+		return XFS_ERROR(EFSCORRUPTED);
+	}
+
 	switch (ip->i_d.di_mode & S_IFMT) {
 	case S_IFIFO:
 	case S_IFCHR: