gfs2: Minor gfs2_write_jdata_batch PAGE_SIZE cleanup

In gfs2_write_jdata_batch(), to compute the number of blocks, compute
the total size of the folio batch instead of the number of pages it
contains.  Not a functional change.

Note that we don't currently allow mounting filesystems with a block
size bigger than the page size.  We could change that after converting
the page cache to folios.  The page cache would then only contain
block-size or bigger folios, so rounding wouldn't become an issue here.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>