There's a spot that computes the number of pages to allocate for a
page-aligned length by just shifting it. Use calc_pages_for()
instead, to be consistent with usage everywhere else. The result
is the same.
The reason for this is to make it clearer in an upcoming patch that
this calculation is duplicated.
Signed-off-by: Alex Elder <elder@inktank.com> Reviewed-by: Josh Durgin <josh.durgin@inktank.com>