This is a fix to help jfs work with grub.  A new IAG is created in
the bd_inode's mapping, but subsequently modified in a different
mapping.  We should invalidate the former page to keep grub from
using that cached page.  It isn't useful to have it cached anyway,
since jfs will never access it again through that mapping.
Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>