jfs: Update agstart when resizing volume

A comment indicates that the IAG's agstart does not need to be updated
since it will always point to a block in the same aggregate group, but
jfs_fsck isn't so forgiving and reports it as an error.

I'm fixing this in jfsutils as well, so either a new kernel or new
utilities will be sufficient to fix the problem.
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>

fs/jfs/jfs_imap.c

@@ -2921,10 +2921,9 @@ int diExtendFS(struct inode *ipimap, struct inode *ipbmap)
 			continue;
 		}
 
-		/* agstart that computes to the same ag is treated as same; */
 		agstart = le64_to_cpu(iagp->agstart);
-		/* iagp->agstart = agstart & ~(mp->db_agsize - 1); */
 		n = agstart >> mp->db_agl2size;
+		iagp->agstart = cpu_to_le64((s64)n << mp->db_agl2size);
 
 		/* compute backed inodes */
 		numinos = (EXTSPERIAG - le32_to_cpu(iagp->nfreeexts))