gfs2: jdata writepage fix

The ->writepage() and ->writepages() operations are supposed to write
entire pages.  However, on filesystems with a block size smaller than
PAGE_SIZE, __gfs2_jdata_writepage() only adds the first block to the
current transaction instead of adding the entire page.  Fix that.

Fixes: 18ec7d5c

 ("[GFS2] Make journaled data files identical to normal files on disk")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>