gfs2: Multi-block allocations in gfs2_page_mkwrite

In gfs2_page_mkwrite's gfs2_allocate_page_backing helper, try to
allocate as many blocks at once as we need.  Pass in the size of the
requested allocation.

Fixes: 35af80ae

 ("gfs2: don't use buffer_heads in gfs2_allocate_page_backing")
Cc: stable@vger.kernel.org # v5.3+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>