[GFS2] Remove unused zero_readpage from stuffed_readpage

Stuffed files only consist of a maximum of
(gfs2 block size - sizeof(struct gfs2_dinode)) bytes. Since the
gfs2 block size is always less than page size, we will never see
a call to stuffed_readpage for anything other than the first page
in the file.

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