gfs2: don't use buffer_heads in gfs2_allocate_page_backing

Rewrite gfs2_allocate_page_backing to call gfs2_iomap_get_alloc and operate on
struct iomap directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>