Apply by doing:
       cd /usr/src/sys
       patch -p0 < bmap.patch

And then rebuild your kernel.

Index: ufs/ext2fs/ext2fs_bmap.c
===================================================================
RCS file: /cvs/src/sys/ufs/ext2fs/ext2fs_bmap.c,v
retrieving revision 1.3
retrieving revision 1.4
diff -u -r1.3 -r1.4
--- ufs/ext2fs/ext2fs_bmap.c    1997/06/12 21:09:31     1.3
+++ ufs/ext2fs/ext2fs_bmap.c    1999/04/25 00:36:46     1.4
@@ -120,7 +120,7 @@
       struct ufsmount *ump;
       struct mount *mp;
       struct vnode *devvp;
-       struct indir a[NIADDR], *xap;
+       struct indir a[NIADDR+1], *xap;
       daddr_t daddr;
       long metalbn;
       int error, maxrun = 0, num;
Index: ufs/ufs/ufs_bmap.c
===================================================================
RCS file: /cvs/src/sys/ufs/ufs/ufs_bmap.c,v
retrieving revision 1.4
retrieving revision 1.5
diff -u -r1.4 -r1.5
--- ufs/ufs/ufs_bmap.c  1997/05/30 08:35:02     1.4
+++ ufs/ufs/ufs_bmap.c  1999/04/25 00:36:47     1.5
@@ -114,7 +114,7 @@
       struct ufsmount *ump;
       struct mount *mp;
       struct vnode *devvp;
-       struct indir a[NIADDR], *xap;
+       struct indir a[NIADDR+1], *xap;
       daddr_t daddr;
       long metalbn;
#ifdef TRACE