JFS: Add back directory i_size calculations for legacy partitions

Linux-formatted jfs partitions have a different idea about what i_size
represents than partitions formatted on OS/2.  The i_size calculation is
now based on the size of the directory index.  For legacy partitions, which
have no directory index, the i_size is never being updated.

This patch adds back the original i_size calculations for legacy partitions.

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>