GFS2: Fix up jdata writepage/delete_inode

There is a bug in writepage and delete_inode which allows jdata files to
invalidate pages from the address space without being in a transaction at
the time. This causes problems in case the pages are in the journal. This
patch fixes that case and prevents the resulting oops.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>