jfs: fix diAllocExt error in resizing filesystem

Resizing the filesystem would result in an diAllocExt error in some
instances because changes in bmp->db_agsize would not get noticed if
goto extendBmap was called.

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Cc: jfs-discussion@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org