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>