JFS: Don't save agno in the inode

Resizing the file system can result in an in-memory inode being remapped
to a different aggregate group (AG). A cached AG number can cause
problems when trying to free or allocate inodes. Instead, save the IAG's
agstart address and calculate the agno when we need it.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>